The most important question not raised during the swine-flu panic could have been asked by a 6-year-old: where do viruses come from? The answer, it turns out, is simple, and scary: viruses come from a giant wellspring of diseases—also known as the environment—that grown-ups should be very careful not to disturb. Pathogens—viruses, bacteria and a wide variety of other parasites—appear in nature as unpredictable, minimalist terrors equipped with little genetic material of their own but the ability to make things up as they go. A bird-flu virus can rest coolly in pigs, then flare up in humans, scrambling genes from all three species in ways impossible to fully anticipate with vaccines. The SARS virus bided its time among palm civets (a kind of mongoose) and horseshoe bats before killing humans in 2002. And possibly the most diminutive of all, the retrovirus HIV emerged from the blood of wild monkeys to become the most efficient destroyer of the human immune system. With strong enough poison and infinitely transmutable genes, a single pathogen could lay deadly siege to the rest of the living world.
The reason this has yet to happen in our lifetimes is that, brilliant as nature is at devising ways to kill, it has also come up with countless ways to cope and survive. Put all the living species together and you have an impressive array of mechanisms to fend off pathogens or contain them in particular ecosystems that have defenses built in. This arrangement, however, is now under serious threat: humans, moving ever deeper into the wild to level forests, extract minerals and plant crops, are changing the balance of ecosystems the world over and taking these defenses apart. These warped ecologies become ground zero for new and deadly infectious diseases, which emerge and spread at an ever-greater rate. This amounts to "Armageddon in slow motion," says Eric Chivian, head of the Center for Health and the Global Environment at Harvard Medical School. Chivian, who shared the Nobel Peace Prize in 1985 for alerting the public to the dangers of nuclear proliferation, now says the danger to human health posed by a degraded planet is "no less devastating than a nuclear war … the ultimate impact might be just as catastrophic."
The evidence is already in. Malaria, currently the most prevalent cause of death in the world, can be ascribed almost entirely to human acts of deforestation, which produces stagnant pools of water and allows more sunlight to reach water surfaces. This intensifies the growth of algae and forms the perfect nursery for Anopheles mosquitoes, potent vectors for the malaria parasite. Anopheles barely had a foothold in the ecosystem in its former state, but when conditions changed—as in the Amazon, East Africa and Southeast Asia—vector mosquitoes quickly displaced other benign species. The spread of other diseases has followed a similar trajectory.
Some snails, for instance, harbor parasitic worms called schistosomes, which infect the human bladder or intestines. In the Senegal River basin, populations of vector snails exploded upon construction of the Diama Dam in 1985, which made the water less saline. The region became a hotbed for schistosomiasis where it didn't exist before; currently, more than 200 million people are victims. Among flies, too, malignant species are winning out: as a result of deforestation, sand flies have surged into human populations in South America and South Asia, infecting millions each year with leishmaniasis, a protozoan parasite that causes skin ulcers and attacks the liver, spleen and bone marrow.
With ecological collapse, the rapid proliferation of disease agents is only half of the gruesome picture; the other is the demise of nonthreatening species. In recent years, disease ecologists Richard Ostfeld and Felicia Keesing have shown how a diversity of species in an ecosystem actually works to suppress infectious diseases. Since not all animals are good reservoirs or vectors for pathogens, the more species there are, the better the chances for a pathogen to be blocked. Ostfeld and Keesing call this the "dilution effect." In healthy ecosystems—say, one with a high diversity of snails or mosquitoes where the dilution effect is strong and infectious disease is better contained—competition from nonvector snails or mosquitoes keeps the vector populations in check.
So the loss of biodiversity is itself a threat to public health, and not only in the deforested Amazon; the denatured suburbs of the United States bear increasing risks, too. Throughout the U.S., the patchy woodlands interspersed among suburban homes are breeding grounds for Lyme disease, a flulike illness that can produce neurological disorders and become impossible to cure. The ideal incubator for the Lyme bacterium is the white-footed mouse, a remarkable survivor in fragmented habitats. Infected mice don't get sick, but they allow the pathogen to multiply and pass it on to ticks who feed on all the local mammals, including humans. Other kinds of forest life—opossums, thrushes, flying squirrels—don't transmit the disease as well to ticks (they're "incompetent hosts"), but fewer and fewer of them remain in the forests. The rising incidence of Lyme disease—27,000 cases in the U.S. in 2007—is a direct result of disappearing forests and the decline of species. "The more nonmouse hosts you have in an ecosystem," Ostfeld says, "the more of the ticks' blood meals will be taken off a host that will not infect them. That will make more of the ticks harmless." Instead, ticks are finding their way to more disease-bearing mice because the mice are increasingly rid of both their competitors and their predators—foxes, weasels, owls. A one- or two-acre scrap of forest poses five times more risk for Lyme disease than a habitat of even five or six acres, with just a few more diverse species.
A similar lack of ecological complexity is responsible for the respiratory disease caused by hantavirus—which has a staggering mortality rate of one in three cases—and the neurological disease caused by West Nile virus. These are not risks limited to bushwhackers and remote rice farmers. West Nile virus landed in New York City in 1999 and, by 2004, reached the West Coast, having found ready reservoir hosts in several common bird species and effective mosquito carriers. "We had vectors, in a sense, sitting in wait," says Ostfeld. "All we needed was the virus to jump the pond." The risk of infection rises as the number of bird species falls: living in a town with more than just the common run of birds—American robins, house sparrows, blue jays and common grackles—makes you 10 times less likely to be infected. But there's a reason that most populated areas don't have much more than the common run, Ostfeld explains. "Those happen to be the bird species that do really well in human-disturbed landscapes."
At this moment, the planet contains an estimated 1,415 pathogens, or disease agents, that we know about: 217 viruses, 307 fungi, 538 bacteria, 66 protozoa and 287 types of worms. Nearly two thirds of these pathogens live among nonhuman species, but they are not staying put: 75 percent of the 175 known infectious diseases now spreading are zoonotic—meaning they have entered the human population via another animal. A new disease comes to light every 12 to 18 months, says William Karesh, head of global health programs at the Wildlife Conservation Society. This is Chivian's Armageddon in the making: any naturally occurring pathogen could prove to be efficient, deadly and infinitely transmutable between species. And it could easily blow up to pandemic proportions. With origins in the recklessly exploited tropics or a remote wetland, it would be an unfamiliar entity to the rest of the world, and as "naive" populations, we instantly become the most vulnerable victims—we would have no defenses.
For Chivian, biodiversity is both the measure of disruption in an ecosystem—the symptom of illness—and the final cost. It makes sense that a less diverse planet would be more sick: biodiversity is also the diversity of defenses, of cures. More simply, biodiversity is not a thing apart from humans. There is no us and them—no useful distinction, when it comes to health, between the human world and the nonhuman. "We share the same pool of infectious diseases as all species," says physician Aaron Bernstein, Chivian's colleague. The absolute worst thing is to end up alone in the pool.