Environmental Issues News
Aug. 2, 2023 — A chemical process used in the browning of food to give it its distinct smell and taste is probably happening deep in the oceans, where it helped create the conditions necessary for life. Known as …
Aug. 1, 2023 — A patch of ocean in the North Atlantic is stubbornly cooling while much of the planet warms. This anomaly — dubbed the ‘cold blob’ — has been linked to changes in ocean circulation, but a …
Aug. 1, 2023 — The closure in January 2016 of one of Pittsburgh’s biggest coal-processing plants led to immediate and lasting declines in emissions of fossil fuel-related air pollutants. These in turn were …
Aug. 1, 2023 — A new study uses nearly a century of data to show that the average heights of winter waves along the California coast have increased as climate change has heated up the …
Aug. 1, 2023 — Nature-based solutions (NBS) can help grand challenges, such as climate change and food security, but, as things stand, communities outside of Europe do not stand to benefit from these innovations. …
July 31, 2023 — Earth is rapidly warming and scientists are developing a variety of approaches to reduce the effects of climate change. An astronomer has proposed a novel approach — a solar shield to reduce the …
July 31, 2023 — Major government and private funding is being invested in planting trees as a powerful tool to fight climate change. But new research shows a troubling bottleneck that could threaten these efforts: …
July 26, 2023 — A new study shows that efforts to reduce methane emissions are needed immediately if we are to meet global climate change goals. A key element of the 2015 Paris Agreement, a legally binding …
July 26, 2023 — Arctic terns — which fly on the longest migrations of any animal on Earth — may be able to navigate the dangers posed by climate change, new research …
July 25, 2023 — Important ocean currents that redistribute heat, cold and precipitation between the tropics and the northernmost parts of the Atlantic region will shut down around the year 2060 if current greenhouse …
July 25, 2023 — For lakes in areas with light to moderate road density, the authors found that holding road salt application rates steady could help lakes stabilize below 230 mg/l of chloride per liter of water, the …
July 25, 2023 — While composting and organic waste ban policies are gaining popularity across the United States, a new study finds dynamic pricing could be the most effective way for grocery chains to keep …
July 24, 2023 — Scientists have determined how the Earth responds as it heats up due to climate change. Their study is the first to find the temperature-carbon dioxide release relationship at the landscape level. …
July 24, 2023 — A study explores effects of Saharan dust clouds on atmospheric methane. Its findings have potentially far-reaching implications for understanding the global methane budget and reasons behind the …
July 24, 2023 — An analysis of more than 202,000 heart attack deaths between 2015-2020 in a single Chinese province found that days that had extreme heat, extreme cold or high levels of fine particulate matter …
July 24, 2023 — New research focuses on optimizing a promising technology called pyrolysis, which can chemically recycle waste plastics into more valuable …
July 22, 2023 — Chemists have demonstrated that water remediation can be powered in part — and perhaps even exclusively — by renewable energy …
July 22, 2023 — Can biosurfactants increase microbiological oil degradation in North Sea seawater? An international research team has been exploring this question and the results have revealed the potential for a …
July 21, 2023 — Changes in ocean wave and storm conditions have not caused long-term impacts on sandy coastlines in the past 30 years, a new study has …
July 20, 2023 — For years, unrecycled plastic bottles have been dumped in landfills. Now, thanks to new research, those bottles may have a second life in that landfill — stabilizing its earth …
Measuring Social Value
Over the last few decades, many people have attempted to measure what is sometimes called social, public, or civic value—that is, the value that nongovernmental organizations (NGOs), social enterprises, social ventures, and social programs create.1 The demand for these metrics has come from all sectors: Foundations want to direct their grants to the most effective programs; public officials, policymakers, and government budget offices have to account for their spending decisions; investors want hard data analogous to measures of profit; and nonprofits need to demonstrate their impact to funders, partners, and beneficiaries. Metrics to meet these needs have proliferated over the last 40 years, resulting in hundreds of competing methods for calculating social value.2
Despite the enthusiasm for metrics, few people actually use them to guide decisions. In the nonprofit sector, good managers are very rigorous about tracking costs and income. But few use sophisticated metrics to help allocate resources. Meanwhile, in the public sector, political judgment counts more than cost-benefit assessments. In the rare cases when decision makers do use metrics of social value, it’s far from clear that they should.
I’ve dealt with social value metrics in a variety of roles: as director of policy and strategy under United Kingdom Prime Minister Tony Blair; as director of the Young Foundation, an NGO that has created dozens of ventures, some for-profit, some social enterprises, and some public; and as an advisor to many other governments. In these positions, I’ve seen not only why social value metrics are ignored, but also how to make them more useful.
One recent project that proved particularly informative was a collaboration between the United Kingdom’s National Health Service (NHS) and the Young Foundation. The NHS commissioned the Young Foundation to develop a practical tool for assessing service innovations and guiding investment decisions. The NHS is a vast organization with a budget of around $150 billion, a workforce of some 1.2 million employees, and contracts with more than 30,000 social enterprises. It needed a set of tools that would be both robust and flexible, and that could be used for decision making as well as evaluation.
We started by scanning existing social value metrics, such as the ones described in the table “10 Ways to Measure Social Value” on page 41. We found hundreds of competing tools, of which foundations and NGOs generally use one set, governments another, and academics yet another. In addition to discovering this segmentation, our survey suggested two more reasons why so few metrics guide real decisions. First, most metrics assume that value is objective, and therefore discoverable through analysis. Yet as most modern economists now agree, value is not an objective fact. Instead, value emerges from the interaction of supply and demand, and ultimately reflects what people or organizations are willing to pay. Because so few of the tools reflect this, they are inevitably misaligned with an organization’s strategic and operational priorities.
The second reason that current measures of social value fail to influence decision makers is that they conflate three very different roles: accounting to external stakeholders, managing internal operations, and assessing societal impact. In the business sector, decision makers use different tools for each of these tasks. An airplane manufacturer, for instance, would use one set of metrics, mandated by laws and regulations, to explain to external stakeholders how it spends its money. The company would then use a second set of metrics to allocate resources in the building of airplanes. (It is a brave manager who would let investors see these internal accounts.) The company would then use entirely different kinds of measures to explain how its activities affect larger economic indicators such as gross domestic product.
Yet in the social and public sectors, some proponents of new social value measures claim that their metric can play all three roles. Not surprisingly, and despite courageous efforts, these attempts to do three things at once have resulted in the failure to do any one of them well.
Here, I describe a better way to think about social value: the product of the dynamic interaction between supply and demand in the evolution of markets for social value. I then show how decision makers in the nonprofit and public sectors can use these insights to measure what can be measured without pretending to measure what can’t be. Finally I recommend better ways to make social value metrics. My main advice is that nonprofits and foundations should resist the current trend of developing assessment tools entirely separately from public policy and academic social science, and instead should collaborate across sectors.
Elusive Quarry
The failure of the social and public sectors to measure the value they create does not stem from a paucity of intelligence or good intention. Rather, it reflects four unavoidable complexities that bedevil the measurement of social value. First among these is the lack of hard-and-fast laws and regularities in the social field. Many people would love the social field to be more like natural science, so that they could definitely predict the effects of, say, a $10 million investment in a crime prevention program.
But unlike molecules, which follow the rules of physics rather obediently, human beings have minds of their own, and are subject to many social, psychological, and environmental forces. Several decades of involvement in evidence-based policymaking has shown me that although evidence should inform all action, very few domains allow precise predictions about what causes will lead to what effects. The social sciences (including business) simply do not have laws in the way that physics has. Even seemingly solid economic principles, such as the rule that demand falls when prices rise, have many exceptions.
A second reason that measuring social value is hard is that, in many of the most important fields of social action—such as crime prevention, childcare, and schooling—people do not agree about what the desired outcome should be. In other words, the public argues not only about social value, but also about social values. For example, many people want to imprison criminals to punish them, even when incarceration costs more and confers fewer benefits than do alternatives to prison. Psychologists call this willingness to sacrifice a lot to penalize others altruistic punishment.
Because people’s ethics, morals, and priorities vary, social value assessments that look only at costs and benefits are bound not to influence many members of the public and the politicians who represent them. Philosophers (from John Dewey to Luc Boltanski) have long recognized that societies are made up of competing and conflicting systems of valuation and justification. But measurers of social value have often tried to deny this.
Even without these problems, many social value metrics are inherently unreliable. Measurements of social return on investment (SROI), for example, often quite arbitrarily estimate costs and paybacks, which dramatically affects the final calculated value. SROI calculations can help in broad-stroke predictions, but they can’t help with finer-grained decisions.
Revealed preference and stated preference methods are also notoriously unreliable. Although they try to provide precise numbers, they are not very rigorous about the means of deriving these numbers. As a result, these methods confuse rigor with precision—a point that REDF and others in the SROI field increasingly recognize.
A final reason that measuring social value is difficult is the problem of time—estimating how much good an action will bring about many years in the future, relative to how much it will cost to implement it now. In predictions of commercial returns on investment (ROI), businesspeople use discount rates to account for the assumption that a given amount of money will be worth less in the future than it is in the present. With a 5 percent discount rate, for example, $100 of today’s money will be worth only $35.85 in 30 years, and only $7.69 in 50 years. Many current measures of social value, such as SROI, likewise use commercial discount rates—perhaps because of a mistaken belief that treating social discount rates as equal to commercial ones will make social value metrics seem more rigorous.
But it’s not clear why social organizations and governments should use commercial discount rates, especially as these rates radically devalue the future. Indeed, we should hope that the people in these organizations give greater weight to the interests of future generations than do commercial markets. A closer analysis of discount rates suggests that they do.3 In health, many countries apply a very low or zero discount rate, on the grounds that younger generations should not be disadvantaged relative to older ones. Governments ignore discount rates when investing in education and defense technologies. And in climate change policy, a furious debate has raged about what discount rates to apply—again in part a moral argument about how to weigh the needs of future generations against the needs of current ones. These examples reflect my broader point: Social value is not an objective fact. Instead, it emerges from the interaction of supply and demand, and therefore may change across time, people, places, and situations.
Constructing Value
Borrowing practices from business and economics has led to many mistakes in the measurement of social value. Yet these fields still offer some important lessons for the field of social innovation. For much of human history, philosophers and economists believed that value was an objective fact. Aristotle thought that there was a “just price” for everything, for instance. And Karl Marx thought that value came from labor.
But more recently, most economists have accepted that the only meaningful concept of value is that it arises from the interaction of demand and supply in markets. In other words, something is valuable only if someone is willing to pay for it. This blunt approach upsets many people because it implies that there may be no economic value in a beautiful sunset, an endangered species, or a wonderful work of art. But this definition of value is useful because it forces economists to observe real behavior, rather than trying to uncover hidden realities.
The time is ripe for the social field to take an equally simple starting point, and to view social value as arising from the interplay of what I call effective demand and effective supply. Effective demand means that someone is willing to pay for a service or an outcome. That “someone” may be a public agency, a foundation, or individual citizens. Effective supply means that the service or outcome works, is affordable, and is implementable. I use the qualifier “effective” because social problems will always invite simple supply and simple demand. But to measure social value, the supplies and demands must be effective in the senses described above.
Markets, conversations, and negotiations then link, on the one hand, people and organizations with needs and resources, with, on the other hand, people and organizations with solutions and services. Social value metrics are useful if they give shape to these markets, conversations, and negotiations.
In some fields, the links between supply and demand are mature. For example, many voters are willing to pay taxes for police forces and primary schools, and many governments are able to supply these services. Likewise, many donors are willing to fund health care for children in developing countries, and many local charities and churches are able to deliver this care. In these domains, analyzing social value is not difficult, because the links between what funders want and what providers know they can offer is clear.
But for other social issues, the links between supply and demand are missing. In some cases, effective demand may be lacking because funders, politicians, or private citizens do not view a need as pressing enough to warrant their resources. For example, some states are unwilling to fund sex education or drug treatment. In other cases, effective demand may be present—for instance, many governments are willing to pay to reduce obesity—but the supply of cost-effective interventions is limited. In these situations any descriptions of social value are bound to be more tentative and exploratory.
In still other cases, both sides of the supply-demand equation may be murky or complex. Many social policy makers, for instance, understand that more holistic solutions often yield better results. But holistic approaches necessarily have to deal with purchasers—that is, demand—that are split across many different public agencies and NGOs, each with its own view of what really counts as valuable. The supply side may also be fragmented: Helping homeless people, for example, may depend on the contribution of many different agencies to provide therapy, alcohol treatment, job training, and housing. In these fields, too, social value can become clearer only through iterative processes that bring together supply and demand in deliberation and discussion. Even the most brilliant researcher cannot measure or even describe social value if she is not immersed in these discussions.
Healthy Numbers
All of these points have become particularly clear in our work for the NHS, which is involved in everything from routine checkups, to surgeries, to behavior-change interventions, to community programs. The advantage of a single, integrated health service like the NHS is that it has to be explicit about its demands, that is, what it needs and what it is willing to pay for. The NHS’s effective supply side is also reasonably easy to define, and it includes doctors, nurses, managers, social enterprises, private providers, and members of the public.
To help the NHS make better decisions and allocate its resources more effectively, we at the Young Foundation created a tool that makes explicit the social value of various alternatives. Earlier on I described the three very different roles metrics can play—external accountability, internal decision making, and assessment of broader social impact. The tool we developed focuses squarely on the second of these goals. It attempts to capture the value that accrues to the individual from being healthy, rather than sick; to caregivers; to the wider community (for example, from the control of infectious diseases); and to the taxpayer.
The tool we created is not a simple computer program or calculator. Instead, it is a framework for thinking about value. Like many of the tools used to assess social value, this one requires a series of judgments. The judgments fall into four main categories: 1) strategic fit (how well the proposed innovation meets the needs of the health service); 2) potential health outcomes (including likely impact on quality-adjusted life years and patient satisfaction); 3) cost savings and economic effects; and 4) risks associated with implementation.
When faced with a proposal, users of the tool apply a 0 to 5 scale to rate the proposal on items in each of these categories. Proposals range from a promising idea from a group of doctors or nurses, to an idea that has already been piloted on a small scale or a venture that is ready for scaling up. Users can also provide commentary along with their ratings.
In some cases, decision makers can draw on strong data—for example, evidence from a randomized controlled trial. In other cases, they must rely on less certain numbers. To capture this variability, the tool also includes measures of the reliability of the evidence on which judgments are based. The visual presentation of the results then makes judgments and their reliability very clear.
Once mastered, the NHS tool is quick to use and transparent. Multiple users can interrogate the judgments, and in due course compare them with what actually happened. It is also publicly available. Ten regional innovation funds (worth around $350 million in total) are using it as a basis for decisions and encouraging applicants to use the tool to assess themselves. Decision makers can also use the tool to review each other’s work, to ensure consistency in their decisions, and to communicate with other public agencies.
The net result of the NHS tool is a picture of social value that is explicit about what’s valued and what isn’t; that doesn’t try to combine everything into a single number; that is transparent and interrogatable; and that is simple enough to help decision makers having to cope with a large volume of examples. It also avoids the flaw of trying to impose a single discount rate onto diverse measurements.
Making Markets
I describe this tool because it is one approach to operationalizing social value that balances coherence, consistency, and simplicity with the flexibility needed to cope with messy and complex phenomena. Developing it was helped by the fact that the NHS is an organization with clear supplies and demands. But for most NGOs, supply and demand are fuzzier, and each field brings with it a different set of concerns.
For example, primary, secondary, and tertiary educational institutions create value for students and the wider society. They rely on a strong research base to decide which types of education deliver which returns to whom. Vocational education, in contrast, presents a different set of considerations. Certain kinds of skill may be of value to only one sector, or to a small set of employers. A program offering intensive support to a chaotic drug user will have a still more complicated set of values, including value for the individual (both financial and health-related), value for the community (for example, from lower crime), as well as value for a wide range of public agencies (from hospitals whose emergency services will be used less, to police, prisons, and welfare agencies).
Seen through this lens, the job of funders is not to alight on one particular method for measuring value. Although common frameworks for thinking about value are useful, funders must adapt these frameworks to the organization and field under consideration.
Indeed, the greatest contribution that funders can make is often not to measure value, but to forge the links between supply and demand that will later generate value. For example, they can invest in effective supply by supporting promising projects and collecting evidence of what works. They can invest in effective demand by persuading governments to use their much greater resources to pay for new services. And they can use their convening power to connect purchasers and providers and then encourage them to talk.
Foundations can also help less powerful players have a voice in the market. Many groups, such as homeless people, migrant workers, and people with mental illnesses, have clear needs but lack the resources and political power to translate their needs into demand. Foundations can turn this latent demand into effective demand. For instance, several European foundations that support undocumented migrants have developed the demand side of this emerging social market by encouraging larger NGOs and public authorities to allocate resources (for example, for housing and health care) to it. On the supply side, these foundations have funded promising projects that are more effective at meeting the needs of this group.
Some foundations are likewise developing the market for addressing elder abuse. On the demand side, they have funded research on the extent of the problem and influenced commissioners to allocate resources and attention to it. On the supply side, they have supported innovative programs to prevent or mitigate abuse.
In both cases, governments’ resources vastly outweigh those of foundations and NGOs. This is almost always the case. Just about anyone wanting to make a big social impact has to engage with the worlds of politics and public provision.
Good Accounts
The field of social innovation can learn some lessons from business and economics. But it should not be naive. As the collapses of Enron and Lehman Brothers revealed, even such seemingly objective metrics as profit are not the facts they appear to be in economics textbooks. And in business, accounts are just that: accounts. They are ways of explaining what is being done, with an eye toward the often conflicting interests of investors, managers, regulators, and consumers. They involve judgments as well as facts.
Anyone who wants to finance social goods and anyone who wants to provide them should use metrics to clarify how inputs can contribute to outcomes, as well as to clarify choices and trade-offs. But they should abandon metrics that obscure these choices or that pretend to offer a spurious objectivity. And they should use metrics only in proportionate ways. It’s not sensible for a small NGO to invest scarce resources in apparently elaborate estimates of social value—not least because these estimates are bound to crumble under serious scrutiny.
Meanwhile, larger NGOs that do need measures of social value should clearly distinguish between those that are primarily about external accountability, those that help internal management, and those that support assessments of broader patterns of social impact. If an organization is using the same method for all three, its findings are almost certainly flawed.
People involved in funding social value, whether at the stage of promising innovations or of large-scale practice, likewise need sharper common frameworks. Greater use of these shared frameworks would be more valuable than proliferation of ever more assessment tools. Building on these frameworks, what matters is the quality of the discussion and negotiation, and the depth of the learning several years later, when participants reflect on what worked and what didn’t.
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Read more stories by Geoff Mulgan.
Social And Environmental Risk Factors In The Emergence Of Infectious Diseases
Popular writing on emerging infectious diseases resounds with dire warnings about the threat of modern ‘plagues’ and losing the ‘war against microbes.’ This adversarial language obscures the fact that most of the microbial world is either neutral toward, or supportive of, human well-being and survival. Indeed, we would not survive long without commensal microbes such as the beneficial strains of Escherichia coli in our gut. That aside, the study of emerging infections is more than a passing fad. The recent rate of identification of such infections, the impact of the SARS outbreak, the devastation caused by AIDS, and the ever-present threat of a new influenza pandemic indicate that we cannot control our disease destiny. Nor are emerging infections unique to humans; the Irish potato famine in 1845 and the English foot-and-mouth disease epidemic in 2001 underscore the consequences for human societies of disease emergence in crops and livestock.
Emerging infectious diseases in humans comprise the following: first, established diseases undergoing increased incidence or geographic spread, for example, Tuberculosis and Dengue fever; second, newly discovered infections causing known diseases, for example, hepatitis C and Helicobacter pylori; and third, newly emerged diseases, for example, HIV/AIDS and SARS.
This Perspective will discuss the human ecology of both the (apparently) new and re-emerging diseases.
The demography of infectious disease
Interest in infectious disease has itself recently re-emerged. In 1972, Burnet and White commented, “The most likely forecast about the future of infectious disease is that it will be very dull. There may be some wholly unexpected emergence of a new and dangerous infectious disease, but nothing of the sort has marked the past fifty years”1. Today, we may criticize the short-sightedness of our mentors’ generation, yet in demographic terms they were essentially correct because the proportion of deaths from infectious disease has fallen throughout the twentieth century2,3 (Fig. 1).Figure 1: Proportions of total deaths from major cause-of-death categories, 1909 and 1999, in Chile2.
This country illustrates the full transition from developing to developed status during the twentieth century.
Humankind currently faces neither apocalyptic extinction nor even a population reduction such as occurred in Europe during the Black Death of the fourteenth century. Rather, overpopulation in relation to environmental resources remains a more pressing problem in many developing countries, where poor economic and social conditions go hand-in-hand with infectious disease. In industrializing countries during the nineteenth century, a major reduction in enteric infections was achieved by separating drinking water from sewage—an environmental change that probably saved more lives than all the twentieth century vaccines and antibiotics together. Today, however, the growth of shanty towns without sanitation around the megalopolis cities of Asia, Africa and South America is recreating similar conditions, and in the past 40 years cholera has made a remarkable re-emergence through its longest ever (seventh) pandemic4.
In most countries, life expectancy has risen over the past 50 years5 (Fig. 2). The most important exception is those regions where HIV infection is rife. Moreover, during the past 15 years, falling living standards in some African countries and the breakdown of public health infrastructure in ex-Soviet nations has aided the re-emergence of transmissible diseases like tuberculosis4,6. Further, severe outbreaks such as the 1918–1919 influenza A pandemic temporarily reversed the decline of deaths caused by infectious disease. The 50 million estimated deaths from that pandemic7 represented about 2% of the global population at that time, and is twice as many as the cumulative AIDS mortality of the past 20 years. The next influenza pandemic may be just around the corner, and may spread even faster8, if access to appropriate vaccines and drug treatment is not available9,10.Figure 2
Changes in life expectancy at birth for both sexes in eight representative countries during the last 55 years5.
For other newly emerging infections that make headlines, such as SARS, Ebola or vCJD, it is important to keep a sense of demographic proportion. Placing these emerging infections on a ‘Richter’ scale of human mortality (Box 1) shows that they elicit scarcely detectable minor tremors in numbers of fatalities — despite the fear they invoke. We do not know, however, which one might leap to the top of the scale like HIV has done; indeed, it may be a completely unknown agent, as the SARS coronavirus was two years ago. A major challenge is to predict which infection presages the next big quake, hopefully allowing preventive action.
Emerging infectious diseases, past and present
Like any other animal or plant species, humans have been prone to infection by pathogens throughout their evolutionary history. Such ancient infections by helminth and protozoan parasites, bacteria, fungi and viruses are endemic, eliciting a range of effects from a heavy burden of disease (e.G., malaria) to being essentially commensal in immunocompetent hosts (e.G., most types of herpesvirus and papilloma virus). Other infections depend on an animal reservoir for their maintenance; their infection of humans may be pathogenic, but it has little part in the evolving ecology of the microbe or parasite. An estimated 61% of the 1,415 species of infectious organisms known to be pathogenic in humans are transmitted by animals11, for which the human represents a dead-end host. Occasionally, however, a zoonotic infection adapts to human-to-human transmission and diversifies away from its animal origin. Epidemic diseases are generally caused by infections that are directly transmissible between humans. HIV is a recent example of a long line of human infections initiated by a switch of host species, stretching back to the origins of measles and smallpox.
Free-living microbes may also find a human niche that suits their lifestyle, such as the lung for Legionella pneumophila and the gut for Vibrio cholerae. Legionnaires’ disease, first recognized in Philadelphia in 1976, is the environmental equivalent of a zoonosis. It is seldom passed directly from person to person but it was human ingenuity in designing warm, aerated, humid ‘artificial lungs’ called air-conditioning systems that allowed the microbe to proliferate and become an opportunistic colonizer of the human lung. Cholera, which was unknown beyond the Ganges delta before it spread widely in Asia and the Middle East during the period 1815–1825, at around that time horizontally acquired a toxin gene and other factors in a genetic package that helped it to colonize the gut; the resultant diarrhea aids dispersal of the microbe12,13.
Human society has undergone a series of major transitions that has affected our pattern of infectious disease acquisition and dissemination4. These transitions illustrate the interrelationship between environmental, social and behavioral influences on the emergence and subsequent spread of infectious disease. Some infections were acquired when our australopithecine ancestors left their arboreal habitat to live in the savannah. This ecological change included exposure to new species of mosquito and tick as vectors for infection. After the emergence of Homo sapiens, the eventual migration of neolithic hunter-gatherers out of Africa 50,000 to 100,000 years ago exposed them to new infections in distant regions.
The first major transition of prehistoric/early historic times gave rise to the epidemic, or ‘crowd,’ infections. This change must have started in the millennia following the advent of agriculture—from around 10,000 years ago—as agriculturally based society developed larger, denser populations. The domestication of livestock and the rich dividends available in human settlements to other animals (e.G., rodents, dogs and various insects) provided further opportunities for pathogens to move between species. Sometimes such a pathogen (or a mutant strain thereof) would have been well suited to humans as a new host species, and, if human numbers were adequate, could therefore persist indefinitely as a human infection. Thus, measles emerged about 7,000 years ago, probably from rindepest of cattle, and diverged to become an exclusively human infection when population size and density became sufficient to maintain the virus without an animal reservoir. Similarly, smallpox became epidemic about 4,000 years ago, possibly evolving from camelpox, its closest phylogenetic relative.
The next two transitions were primarily to do with great extensions in the spread of infectious diseases, entering distant populations as ‘new infections.’ Thus, the second historical transition occurred in Classical times as large Eurasian civilizations came into commercial and military contact. They inadvertently exchanged their pools of infections, and vectors such as rats and fleas, across the Mediterranean basin, the Middle East, India and China. The plague of Athens in 430 BCE during the Peloponnesian war vividly described by Thucidides may represent the first report of typhus. This Rickettsial infection is transmitted from rats to humans and thence among louse-ridden humans. Typhus frequently accompanies human conflict and deprivation, as seen in a recent outbreak among Rwandan refugees in Burundi14. The Justinian plague of 542 CE devastated the eastern Mediterranean region and probably extended as far as China15 like the Black Death 800 years later (and both are attributable to Yersinia pestis16).
The third historical transition accompanied the era of worldwide exploration and colonization by Europeans from circa 1500 CE onward. A contemporary account by one of Hernan Cortes’ fellow conquistadors, Bernal Diaz, recalls that they might well have failed to overthrow the mighty Aztec empire had they not been aided by a raging epidemic. This was possibly a combination of smallpox and measles, both wholly unknown to the New World population. Curiously, the Columbian exchange was unidirectional regarding infectious diseases; the one contentious possible exception being syphilis. The New World is believed to have had substantially fewer human zoonotic infections15,17, and vector-borne infections like Chagas’ disease did not travel in the absence of an appropriate vector.
Two centuries later, Captain Cook unwittingly repeated the decimation of indigenous peoples through syphilis, measles and tuberculosis in many of the Pacific islands, whereas Lord Jeffrey Amherst deliberately attempted to spread smallpox among ‘hostile’ Native Americans, one of the better documented cases of germ warfare18. The transmission dynamics of infections in naive populations is markedly different from those in which the majority of adults are immune19. Onboard The Beagle, Charles Darwin observed with his customary acuity, “Wherever the European has trod, death seems to pursue the aboriginal …Most of the diseases have been introduced by ships and what renders this fact remarkable is that there might be no appearance of the disease among the crew which conveyed this destructive importation.”
Today we are living through the fourth historical transition of globalization. Urbanization, dense and usually impoverished peri-urban settlements, social upheaval, air travel, long-distance trade, technological developments, land clearance and climate change all influence the risks of infectious disease emergence and spread. Although some of the apparent increase in infectious disease may be attributable to better diagnostic methods and surveillance, there seems little doubt that more incidents are occurring, and have the potential to spread more widely than 50 years ago, as outbreaks and spread of infections like Nipah virus and SARS would not have passed unnoticed.
Environment and emerging infectious diseases
As humans encroach further into previously uncultivated environments, new contacts between wild fauna and humans and their livestock increases the risk of cross-species infection20. This process will only diminish as wild species become rarer and eventually endangered, like the great apes today. An example of such contact followed the establishment of piggeries close to the tropical forest in northern Malaysia, where, in 1998, the Nipah virus first crossed over from fruit bats (flying foxes, Pteropus spp.) to pigs and thence to pig farmers21. Destruction of natural forest has also encouraged fruit bats to relocate nearer human habitation, like the large colony in the botanic gardens in the heart of Sydney. Indeed, in 1997, Hendra, a related paramyxovirus of Australian fruit bats22, fatally infected a veterinarian examining a sick horse.
Rodents continue to be sources of re-emerging infections, as witnessed in the 1990s with hantaviruses in the United States. Rodent-borne hantavirus is prevalent in agricultural systems in South America and East Asia, in arid grasslands in North America and elsewhere. In mid-1993, an unexpected outbreak of acute, sometimes fatal, respiratory disease occurred in humans in the southwestern United States23. This ‘hantavirus pulmonary syndrome’ was caused by a previously unrecognized virus, maintained primarily within the native deer-mouse, and transmitted through excreta. The 1991–1992 El Niño event, with unseasonal heavy summer rains and a proliferation of piñon nuts, hugely amplified local rodent populations which led to the 1993 outbreak23,24. In South America, there have been several outbreaks of hantavirus and arenavirus infections linked to forest clearance and the growth of rodent populations in the new grasslands4.
Habitat destruction is not the only cause of increased human infection, however. Dengue virus is extending its range and prevalence because its mosquito vector breeds rapidly in the urban environment25. In the United States, nature conservation and increased woodland in the eastern states has led to the emergence of Lyme disease. This disease is caused by a tick-borne spirochete and the presence of tick-infested deer near suburban homes leads to ticks residing on bushes adjacent to baseball diamonds and gardens.
Food-borne infections
Intensification of production of meat and meat products has led to new infections26. Most notorious is vCJD in the UK arising from consumption of contaminated food products of cattle affected by BSE27. BSE, or ‘mad cow disease,’ emerged in British cattle in 1986 because of industrialized cannibalism, whereby rendered neural tissue and bone meal from slaughtered cattle were recycled into cattle feed, as well as into pies and hamburgers for human consumption28. Originally, infectious prions from scrapie in sheep were the suspected source, but it now seems more likely that it arose from a bovine with sporadic prion disease. The extent of the human epidemic remains unclear. Although natural transmission is unsustainable (R0 < 1 in both cattle and humans), there are concerns that vCJD might be transmissible through blood transfusions29. Without effective diagnostic tests for presymptomatic vCJD infection, this situation is extremely unfortunate.
Other recently emergent food-borne infections include E. Coli O157:H7, which is harmless to cattle but toxic to humans, and Salmonella enteriditis in chickens. Better hygiene in abattoirs, butchers and domestic kitchens can greatly reduce the incidence of infection. In theory, closed and intensive farming of a single species should reduce the risk of cross-species infection (Fig. 3). But it also allows large-scale epidemics to emerge, as seen recently for avian influenza strains in southeast Asia and the Netherlands8,30.Figure 3: The changing pattern of farming in South East Asia.
REUTERS/Dadang Tri en/TW INDONESIA CIANJUR
Top, traditional mixed homestead; bottom, intensive single-species industry. (Top photo courtesy of R.A. Weiss)
Ancient dietary taboos, such as those of Hindus, Muslims and Jews regarding pork as unclean, doubtless had their roots in protection from infectious disease. Today, an increasing demand for consumption of exotic and wild animals raises new risks of infectious diseases such as SARS (Box 2).
Transmissibility and globalization
Changing patterns of human behavior and ecology affect two distinct steps in the emergence of new infectious disease. The first is an increased opportunity for animal-to-human infection to occur owing to greater exposure, which may be necessary but not sufficient to lead to the emergence of a new human infection. The second step is the opportunity for onward transmission once a person has become infected. For each novel epidemic, such as the 1918–1919 influenza pandemic or AIDS, there are probably thousands of failed transfers.
Some infections simply do not take in the new host. Innate host-specific restrictions on viral replication have recently become evident for primate lentiviruses31, which may explain why certain species that harbor simian immunodeficiency virus, but not others more commonly in contact with humans, gave rise to HIV-1 and HIV-2. Even in the case of HIV-1, only one pedigree of three independent chimpanzee-to-human crossover events32 has given rise to the AIDS pandemic, whereas the other two smolder as poorly transmissible infections.
Fatal pathogenesis is not necessarily coupled with infectiousness12, which is evident for H5N1 avian influenza in humans9. But genetic reassortment between avian and human influenza viruses could easily give rise to a new, rapidly spreading strain8. A poorly infectious pathogen may not spread at all from the index case, as is usual with rabies, or may only infect close contacts and soon peter out, as seen with Lassa fever and Ebola virus. SARS nearly became self-sustaining but was brought under control. Some of the most insidious infections are those with long, silent incubation periods during which the person is infectious. These emerge surreptitiously so that when the new disease is eventually recognized, as AIDS was in 1981, the infection has already spread far beyond control.
Like the ships of centuries past, the speed of modern air travel works wonders for the dispersal of infectious diseases. SARS was eventually constrained by quarantine and strict adherence to infection control guidelines in hospitals, but not before it quickly traveled from Guangzhong to Hong Kong and on to Toronto. If Ebola broke out in a city with a busy international airport, it might also travel across continents in a similar manner. Brockmann33 has modeled how rapidly such infections can move once they reach a major airport hub; closing the hubs becomes an immediate imperative. We cannot be sure what the initial vector was for the arrival of West Nile virus into North America in 1999: a migratory bird blown off course, an infected human with a valid air ticket or a stowaway mosquito on a similar flight. Whatever the means of entry and early colonization of crows in New York, it has taken less than four years to reach the Pacific coast25. Thus, West Nile virus has found a new reservoir in American birds, just as yellow fever virus reached New World primates 350 years earlier.
Social and economic conditions, behavioral changes and geopolitical instability
Microbes frequently capitalize on situations of ecological, biological and social disturbance. Biologically weakened and vulnerable populations—especially if also socially disordered and living in circumstances of privation, unhygienic conditions and close contact—are susceptible to microbial colonization. The severity of the bubonic plague (Black Death) in mid-fourteenth-century Europe seems to have reflected the nutritional and impoverishment consequences of several preceding decades of unusually cold and wet weather with crop failures compounding the incipient destabilization of the hierarchical feudal system.
Many of the rapid and marked changes in human social ecology in recent decades have altered the probabilities of infectious disease emergence and transmission. These changes include increases in population size and density, urbanization, persistent poverty (especially in the expanding peri-urban slums), the increased number and movement of political, economic and environmental refugees, conflict and warfare. Political ignorance, denial and obduracy often compound the risk of infectious disease transmission—as has been tragically observed with HIV/AIDS in parts of Africa, where widespread poverty, a culture of female disempowerment and political instability further exacerbate the problem34,35. But we have little understanding of why the prevalence of HIV infection varies so greatly between cities in sub-Saharan Africa36.
The urban environment has only recently become the dominant human habitat. Urbanism typically leads to a breakdown in traditional family and social structures, and entails greater personal mobility and extended and changeable social networks. These features, along with access to modern contraception, have facilitated a diversity of sexual contacts and, hence, the spread of sexually transmitted diseases37. This risk is further amplified by the growth in sex tourism in today’s internationally mobile world, which capitalizes on the desperation and ignorance of poverty, combined with exploitative behaviors, in developing countries. More generally, cities often function as highways for ‘microbial traffic’38. Rapid urbanization boosts certain well established infectious diseases, such as childhood pneumonia, diarrhea, tuberculosis and dengue, and facilitates dissemination of various ’emerging’ diseases—as occurred for SARS in the high-rise housing of Hong Kong. Crowded and dilapidated public housing can potentiate infectious disease transmission through drug abuse and sexually transmitted infections39.
Nosocomial and iatrogenic infections
Technological advances in medicine and public health can also inadvertently promote the emergence and spread of infectious disease. It has become commonplace to quip that you go to the hospital at the peril of acquiring an intractable nosocomial infection such as methicillin-resistant Staphylococcus aureus40, and such infections killed around 40 times as many people as SARS did in 2003 (Box 1). Multidrug-resistant tuberculosis has also become a major problem, and, paradoxically, regions with health programs that reduced wild-type tuberculosis strains can develop into ‘hot zones’ for multidrug-resistant tuberculosis41.
By far the most effective medical vector of infectious disease has been the syringe and needle. Drucker et al.42 have charted the massive increase in the use of injecting equipment over the past 100 years. Individuals with hemophilia treated with pooled clotting factors became almost universally infected with hepatitis B and C viruses before diagnostic screening tests were developed. Over 20% of such affected individuals also became infected with HIV43, and more recently, transmission of West Nile virus by blood transfusion and by organ transplantation has been reported44,45. The use of contaminated needles among intravenous drug users has had similar consequences. Infectious diseases have also been amplified by the use of nonsterile medical injections in developing countries42. Egypt has the highest prevalence of hepatitis C infection in the world because of the use and reuse of syringes and needles in an earlier public health campaign to reduce bilharzia by medication given by injection. The transmission of CJD through contaminated surgical instruments is another example of iatrogenic spread of infection29.
Biological medicines produced from animal-cell substrates present an inherent potential hazard for introducing new infections. Great care must be taken to ensure that live attenuated vaccines grown in animal cells or eggs are devoid of pathogens; for example, several early batches of live and inactivated polio vaccine unwittingly contained live SV40 virus, a polyoma virus of macaques. After SV40 was discovered in 1960, polio vaccine production shifted to virus propagation in primary kidney cells of African green monkeys. These cultures were free of SV40 but possibly contained SIVagm, a relative of HIV that fortunately does not infect humans31. The irony of the SV40 story is that the United States Food and Drug Administration prohibited the use of well known, permanent cell lines demonstrably free of adventitious infectious agents, for fear that such immortalized cells might exert oncogenic properties on the vaccine. There is no epidemiological evidence of increased tumor incidence in those populations who are known to have received SV40-contaminated polio vaccine. But there have been a number of recent claims of an association of SV40 DNA sequences in a variety of human malignancies46, although these findings remain controversial47.
The ultimate medical means of introducing animal viruses into humans is xenotransplantation. The implantation of animal cells or tissues into immunosuppressed individuals seems to be a perfectly designed way to encourage cross-species infection. It is astonishing that trials were started without much thought about the consequences for potentially emerging pathogens, for example, porcine retroviruses48. The generation of genetically modified knockout or transgenic animals to prevent hyperacute rejection of donor tissues may exacerbate the infection hazard49,50. Happily, there is no evidence so far of retrovirus infection in individuals who were exposed to living pig cells50, and clinical xenotransplantation is now stringently regulated; so it seems all the more extraordinary that cellular therapies with fetal lamb cells and extracts continue to be practiced with impunity in alternative medicine clinics in Europe and the Far East.
Conclusions and prospects
Novel infectious diseases can emerge in any part of the world at any time. HIV and Ebola came out of Africa, avian influenza and SARS from China, Nipah virus from Malaysia, BSE/vCJD from the UK and hantavirus pulmonary syndrome from the Americas. It is difficult to predict what new disease will come next or where it will appear, but changing ecological conditions and novel human-animal contacts will be useful clues as to which horizons require scanning with most scrutiny. We must expect the unexpected.
As a codicil, another factor that needs to be taken into account is the potential impact of the HIV pandemic on the emergence of other infectious diseases51. We already know that persons with AIDS act as ‘superspreaders’ of tuberculosis, and we can only speculate what course the SARS outbreak might have taken had someone incubating the disease flown to Durban rather than Toronto52. People with AIDS may persistently harbor infections that would otherwise be transient, and this could hamper the eradication of measles and polio. Multivalent Pneumococcus vaccines are ineffective in HIV-infected people with CD4+ lymphocyte levels below 200/μl, whereas live ‘attenuated’ vaccines such as vaccinia can cause virulent disease in the immunocompromised host. Immunodeficient persons living at high density could also be the seed-bed for microorganisms that are initially ill adapted to human infection to evolve into transmissible human pathogens. Thus, an infection from a zoonotic or environmental source—for example, the Mycobacterium avium intracellulare complex—could conceivably emerge as the tuberculosis of the twenty-first century, although direct transmission between individuals with AIDS of such opportunistic infections have not been documented so far.
We shall give Girolamo Frascatoro the last word on emerging and re-emerging infectious diseases by quoting from his treatise De Contagione, published almost 450 years ago, “There will come yet other new and unusual ailments in the course of time. And this disease [syphilis] will pass way, but it later will be born again and be seen by our descendents.”Box 1: Natural weapons of mass destruction placed on a ‘Richter’ scale52,53.
The values are approximate global death rates for the year 2003, taken from the World Health Organization (WHO) and other sources. HBV and HCV, hepatitis B and C viruses; RSV, respiratory syncytial virus; HPV, human papilloma viruses; vCJD, variant Creutzfeldt-Jakob disease.
Two major, novel causes of mortality top the list: cigarette smoking and HIV infection; they emerged in the twentieth century and continue to increase in many developing countries. Among the chronic and re-emerging infections, malaria and tuberculosis are near the top, so it becomes apparent why there is a need for the Global Fund for Malaria, Tuberculosis and AIDS. Accidental injuries, particularly road deaths, continue to rise, with 85% occurring in developing countries54. Although 2003 was the year of the SARS outbreak52,55, less than 1,000 people actually died as a result of SARS coronavirus infection despite the collateral damage to daily life, psychological well-being and economic activity in the affected cities.
This Richter scale represents a snapshot in time. Twenty years ago, HIV was three logs further down the scale, whereas polio was three logs higher. Fifty years ago, malaria was finally eradicated from Europe, where it had formerly been widespread, including in England (Shakespeare’s ‘ague’). Bacterial respiratory diseases used to have a more important role in human mortality and, despite concern over multi-resistance to antibiotics40,56, the situation is considerably better than in the era before the advent of antibiotics. Common bacterial infections of childhood, such as diphtheria and whooping cough, have become rarities in the developed world, largely through vaccination. Viral diseases have similarly been reduced. Thanks to effective immunization policies of the WHO, smallpox was eradicated in 1977; polio and measles viruses, which have no animal reservoir, may soon be eliminated in the same way.Box 2: Bushmeat and live animal markets
“If there is any conceivable way a germ can travel from one species to another, some microbe will find it,” wrote William McNeill in his classic text Plagues and Peoples15. For millennia, small farmsteads accommodated mixed species living closely with humans—goats, pigs, cattle, ducks, geese, chickens and perhaps a water buffalo or a donkey—and exchanged infections. When species are raised separately but are sold together, the opportunity for cross-infection moves from the farm to the marketplace. The 1997 outbreak of avian influenza in Hong Kong occurred in mixed markets, where live chickens, quail and ducks were stacked together in close quarters with humans. The H5N1 virus that emerged may have been derived by recombination between those of different avian hosts8. After 1997, mixed species were separated into different areas of the markets. But this year’s H5N1 virus is spreading among intensively reared chickens across southeast Asia.
The increasing predilection for meat of exotic species has exacerbated the risk of exposure to infections not previously encountered, and this situation probably triggered the SARS epidemic55. Although we are still not sure of the natural reservoir species of SARS coronavirus, the live markets and restaurants in Guangzhong sold small carnivores, and several species of civet cat, racoon dog and ferret badger captured in China, Laos, Vietnam and Thailand, were brought into close proximity57. Clearly, some of the palm civet cats were infected with SARS-related viruses, but it is less clear whether they represent the original source species. There is a danger in incriminating the wrong species; if the true reservoir resides in the rodent prey of these carnivores, then culling the predators may be counterproductive. Stopping the exotic meat trade altogether would seem to be a simple solution to prevent the reappearance of SARS, but once the taste for it has been established, that may prove no more practical than attempting to prohibit the tobacco trade.
In Africa, bushmeat also poses a serious problem for emerging infectious diseases, as well as for nature conservation. Sick animals may be more easily captured. For example, 21 human deaths owing to Ebola virus infection ensued from the butchering of a single chimpanzee58. HIV has crossed from chimpanzees to humans on at least three occasions, and a higher number of zoonotic events from sooty mangabeys are indicated for HIV-2 (ref. 32). Whether these cross-species infections arose from butchering the animals or from keeping them as pets is unknown, but a recent survey of primate hunters in Africa showed that they are susceptible, like handlers of primates in captivity, to infection (though not disease) from foamy retroviruses59.
The escalating intercontinental trade in exotic pets can lead to unexpected infectious disease outbreaks. The United States has only recently imposed more stringent regulations and quarantine following cases of monkey pox in humans and in prairie dogs introduced by rodents imported from Africa as pets60.