Today, old familiar scourges like tuberculosis, malaria, measles, and diarrhea -- and a newer one, AIDS -- are the world's biggest killers, but they've been joined by a host of newcomers. Indeed, one could get the impression that each year brings a new disease. That's because it does.
Mark Woolhouse, chair of Infectious Disease Epidemiology at the University of Edinburgh, has counted 38 new pathogens (disease-causing biological agents) that have moved into the human population from other animal species in just the past 25 years. In a presentation at the annual meeting of the American Association for the Advancement of Science last month, Woolhouse noted that we're under assault not only from those novel species, but also from new genetic variants of pathogens that have been with us for a long time.
A recent tally identified 1,415 disease-causing microbes in humans, including bacteria, viruses, fungi and parasitic worms. We share fully 61 percent of those pathogens with other animal species. Of the total, 175 cause "emerging diseases" -- ones not known until recently in humans. Of those, 75 percent came out of other animals to invade Homo sapiens.
The impact of species-jumping pathogens varies. Hendra virus moved from fruit bats to horses in 1994 and is known to have killed a total of only three people. Since the 1970s, the Ebola virus has incited some horrifying outbreaks that, so far, have failed to blow up into epidemics. Influenza viruses usually cause a lower mortality rate but hit far more people; currently, an H5N1 "bird flu" strain threatens to break that pattern by staging an encore of the 1918-19 killer flu pandemic that killed 50 million to 100 million people. HIV/AIDS is both chronically widespread and deadly, now accounting for almost a fourth of infectious disease deaths.
But have "emerging" species-jumping diseases actually been with us for millenia, identified only when medical research achieves sufficient precision in detecting and identifying microbes? Durland Fish, professor at the Yale School of Public Health, says that better research is part of it, but there still appears to be a faster rate of disease appearance these days. He told me, "Dr. Woolhouse makes an interesting point: that 'emerging disease' is a new concept but a very old process. Humans have always acquired new diseases." We're being hit more frequently today than in previous eras, he says, partly because "transportation, trade, human population growth, and environmental change are going on at unprecedented rates."
They don't show up uninvited
Scientists have seen associations between human activites, which have burgeoned in the past quarter century, and diseases that gained prominence during those same years. Some examples:
The chances of the potentially catastrophic flu virus H5N1, and others like it, emerging from interaction between wild birds, domestic animals and people may have been enhanced by loss of natural wetlands in southern China. That has led infected migratory birds to alight more often on farms and other populated areas. There, they come into contact with denser populations of chickens, ducks and pigs destined to satisfy an increasing rate of animal-protein consumption per person.
Ticks transfer the bacterium that causes Lyme disease from infected mice and deer to people. First described in New England in the 1970s, Lyme disease is now a chronic problem in parts of the United States. Reforestation in eastern states, but by a less diverse ecosystem than the one that was destroyed during original white settlement two to three centuries ago, has brought large populations of deer, mice and ticks into much closer contact with suburb-dwelling humans.
Mad cow disease is believed to have resulted from the ecologically suspect practice of feeding processed livestock remains to naturally vegetarian cattle. Scary little protein fragments called prions that appear to be responsible for the disease are not destroyed when meat is cooked. They can and do strike humans, causing the debilitating and inevitably fatal condition known as variant Creutzfeldt-Jakob disease.
Perhaps the biggest threat from new genetic strains of old, familiar pathogens is the onslaught of bacteria resistant to multiple antibiotics. Livestock are now a widely recognized source of drug-resistant strains of salmonella, E. coli and other bacteria. Heavy feeding of antibiotics to cattle, swine and poultry (often even when they're not sick) in the overcrowded, filthy conditions of gigantic feedlots, animal-confinement facilities and meat-packing plants provide ideal incubators for bacteria resistant to the drugs. Meat coming out of such ecological horror houses can contain animal feces bearing the newly evolved "superbugs." Of 10 organisms listed by the U.S. Public Health Service as the most serious threats in this country, seven are carried by meat and dairy products.
The most catastrophic of the recent emerging diseases so far has been AIDS. The route by which HIV jumped to humans is still a matter of speculation, but encroachment into forests and the resulting increased contact with other primate species is widely believed to have been involved.
Destruction of forest habitat in Asia has driven several species of fruit bats infected with Nipah virus into increasing contact with pigs and humans, and both are susceptible. Nipah is an especially nasty virus, causing severe headache, fever, nausea and seizures. In seven outbreaks since 1999, in Malaysia, India and Bangladesh, it has killed one-third to three-fourths of its victims. A series of Bangladesh cases in 2004 indicated possible human-to-human transmission -- an ominous sign.
When severe acute respiratory syndrome (SARS) broke out in Guangdong province of China in 2002-03, the virus that causes it was also found in palm civets. Soon, the small wild mammals -- a traditional food in the province -- were being targeted for killing by the thousands. Some scientists decried the slaughter as unnecessary ecological disruption. Now it appears that bats, not civets, are the reservoir for the virus. And experts are saying that bat extermination programs would be no more effective than civet killing as a way to curb SARS, Nipah or other bat-harbored diseases.
When people in the American Midwest began falling ill in 2003 with monkeypox (a disease similar to human smallpox), investigators quickly discovered that all of the victims had been in contact with that beloved North American native, the prairie dog. The pox virus had entered the country in infected African rodents legally imported by pet stores, where they had passed it on to the highly susceptible prairie dogs.
Since its first detection in the United States -- in New York in 1999 -- West Nile virus has become an annual threat in many U.S. regions. The virus kills 5 percent to 15 percent of those infected, and more than one-third of elderly patients who are infected die from it. It's known to infect more than 200 species of birds, but unlike bird flu, it doesn't depend primarily on migratory fowl to get around. It's passed to humans by mosquitoes, and many human activities make mosquito populations more mobile. For example, the Asian Tiger mosquito, one of at least 43 species known to carry West Nile, has been reaching U.S. ports since the 1980s in water that collects in used tires imported from Asia. However, it is still not known how the virus first reached this country. (One also wonders why we're importing used tires.)
You play, you pay
Modern human plagues aren't a result of mysterious forces. It's not, as Kurt Vonnegut has put it, that "the Earth's immune system is trying to get rid of us." Diseases have concrete, often mundane causes. The necessary species and genetic variants are everywhere, and whether we mean to or not, we're relentlessly seeking them out and inviting them to do their worst.
To cause human disease, a pathogen first has to come into contact with people. As with bird flu, Nipah and lyme disease, environmental disruptions like habitat destruction or distorted reforestation serve that purpose well. Or, as with monkeypox and SARS, the exotic-pet or exotic-food industries can introduce pathogens to their new home.
However, every new disease-of-the-year doesn't blow up to the catastrophic scale of HIV, which was first recognized two decades ago and is now estimated to be killing almost three million people a year. The impact of most new diseases is ghastly for victims but very small for humanity as a whole. How do a few microbial species go on to cause widespread illness and death, while others don't?
Like any organism entering a new environment, the microbe population either must have within it some genetic variants that are somewhat well-adapted to their new human host, or, once in the host, it has to throw up new, better-adapted forms quickly through mutation or by scavenging genetic material from other strains or species. That's probably why a large proportion of new human diseases are RNA viruses, which mutate and scavenge more readily than DNA viruses, bacteria or other pathogens.
Chance mutations that improve an organism's ability to thrive are extremely rare, even among viruses. This year, the world is watching and waiting to find out if the H5N1 bird-flu strain is capable of producing mutants that can spread directly from person to person. Two years ago, we were wondering if SARS would beat the odds and go global. But it's not all up to the pathogen; as its hosts, we help determine its success.
Given enough opportunities, even highly improbable events have a way of eventually happening. Twenty-first-century humanity does everything in a big way, and much of what we do gives microbes the multiple chances they need to make the improbable unavoidable. Rare, better-adapted genetic combinations may not succeed in the first or fifth or 50th person they've infected -- but give them enough opportunities, and they'll be off and running.
In a 2005 paper published in the journal Trends in Ecology and Evolution, Mark Woolhouse and two colleagues described the mathematical hurdles a species-jumping germ must clear before it can sort out or produce the necessary mutants, spread through a population and cause an epidemic. The lucky pathogen that finds itself in a human body gets a boost over those hurdles, because of the sheer scale of civilization.
When people crowd into high-density cities, sprawling slums and hospitals; consume insufficient or bad food and polluted water; travel widely and often; ship vast quantities of products worldwide; make sex an industry; damage their immune systems by disease, chemotherapy, transplant-facilitating drugs or environmental toxins; or are plunged into the chaos of war, the pathogen has a much bigger field of play.
Some efforts to economize through greater resource efficiency can give pathogens the boost they need. In "The Coming Plague," Laurie Garrett noted that in the 1980s, airlines began saving fuel by drastically cutting the rate of cabin air turnover, and that large numbers of people now live and work in "energy efficient settings" that also restrict outside air flow. Groups of people repeatedly rebreathing the same air have a better chance of getting sick.
Some pathogens, like West Nile virus, don't have to work out a genetic system for direct person-to-person transmission because they've evolved to be transferred by mosquitoes or other vectors. And any ecological disruption that creates favorable conditions for disease-carrying species of insects or ticks favors the disease as well.
It's not surprising that descriptions of humanity's attempts to fight off microbial assaults often involve military imagery. In his 2001 book, "War and Nature," Edmund Russell describes how malaria-laden mosquitoes were often equated with America's Japanese enemies in World War II-era propaganda, while pesticides used to fight the insects had originated in the chemical weapons industry. Our war with mosquitoes has produced no winner. Despite the vast quantities of insecticides sprayed in the years since, malaria still kills 1.2 million people a year.
And then there's global warming, the grandaddy of all ecological threats. What effect will it have on human disease? Many predictions are dire, because warmer conditions have the general effect of increasing biological activity. There is concern, for example, that tropical insect species will bring pathogens into now-temperate regions.
Yale's Durland Fish downplays the specter of pestilence: "We don't have a lot of convincing evidence that global warming will result in epidemics. So far, health alone is not a sufficient reason to reduce CO2."
The overall forecast may indeed be cloudy, but for specific diseases there is very good evidence that more people would fall sick in a hotter world. Outbreaks of cholera in Asia and Latin America have been shown to happen when coastal ocean temperatures rise, as they do during El Niño events. Cholera bacteria lying dormant in the bodies of microscopic marine animals called copepods are stimulated by the warmer temperatures to become active, multiply rapidly and cause local outbreaks.
Heading off future pandemics
The modern better-living-through-chemistry approach is unlikely to do us much good in the face of new pathogens or new, more virulent forms of old ones. Especially against viruses, existing drugs are rarely very effective, and pharmaceutical companies are unlikely to make the huge investment of time and money to develop new drugs until a new disease is already a widespread problem.
As Laurie Garrett explained last year in a comprehensive review of the bird flu threat, we should not expect companies to develop an effective vaccine in time. For one thing, vaccines are much less profitable and more risky than are drugs. In a more recent piece, Garrett argued for a public-health approach that involves monitoring wild bird and virus movements and protecting domestic fowl when the virus is expected to hit a particular area. She even called on the world's bird watchers to help in the effort.
As long as our species continues making the planet a friendlier place for microbes that can infect us, we'll never see the end of potential public-health emergencies. When I asked Durland Fish if he was placing bets for or against a bird flu pandemic, he wouldn't venture any guesses; rather, he made this prediction: "Sooner or later, whether it's H5N1 or another strain, a pandemic is inevitable -- like an earthquake in California."
When new diseases show up, we have no choice but to deal with them. But in the meantime, we need to reverse the ecological damage that makes us increasingly vulnerable. Doing that would also help reduce the already huge and largely unnecessary death toll from existing infectious diseases. That toll currently stands at about 12 million per year worldwide, chiefly in the most severely impoverished parts of the planet. The World Health Organization weighed in late last year with its contribution to the global Millenium Ecosystem Assessment project. In a summary of its report, WHO saw people's health as closely tied to the health of the planet:
Measures to ensure ecological sustainability would safeguard ecosystem services and therefore benefit health in the long term. Where a population is weighed down by disease related to poverty … the provision of [shelter, food and water] should be the first priority for public health policy. Where ill health is caused, directly or indirectly, by excessive consumption … substantial reductions in consumption would have major health benefits while simultaneously reducing pressure on life-support systems.
This is not a "war" that can be "won." We can't command viruses to stop swapping RNA or order birds to stop migrating. And among the many species that humans are known to be driving to extinction, none are microscopic. No matter how cruel some of those microbes can be when they manage to invade our bodies, the only long-term answer is to live and let live.
Stan Cox is a plant breeder and writer in Salina, Kan.
Infected Planet- Modern human plagues like bird flu aren't the result of mysterious forces. Whether we mean to or not, we bring them on ourselves.
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