centuries of efforts at coping with their effects, concerted international attempts to eradicate or control them, and a pretty clear scientific understanding of how they work. Others are new and inexplicably sporadic, claiming a few victims (as Hendra does) or a few hundred (Ebola) in this place or that, and then disappearing for years.
Smallpox, to take one counterexample, is not a zoonosis. It’s caused by variola virus, which under natural conditions infects only humans. (Laboratory conditions are another matter; the virus has sometimes been inflicted experimentally on nonhuman primates or other animals, usually for vaccine research.) That helps explain why a global campaign mounted by the World Health Organization (WHO) to eradicate smallpox was, as of 1980, successful. Smallpox could be eradicated because that virus, lacking ability to reside and reproduce anywhere but in a human body (or a carefully watched lab animal), couldn’t hide. Likewise poliomyelitis, a viral disease that has afflicted humans for millennia but that (for counterintuitive reasons involving improved hygiene and delayed exposure of children to the virus) became a fearsome epidemic threat during the first half of the twentieth century, especially in Europe and North America. In the United States, the polio problem peaked in 1952 with an outbreak that killed more than three thousand victims, many of them children, and left twenty-one thousand at least partially paralyzed. Soon afterward, vaccines developed by Jonas Salk, Albert Sabin, and a virologist named Hilary Koprowski (about whose controversial career, more later) came into wide use, eventually eliminating poliomyelitis throughout most of the world. In 1988, WHO and several partner institutions launched an international effort toward eradication, which has succeeded so far in reducing polio case numbers by 99 percent. The Americas have been declared polio-free, as have Europe and Australia. Only five countries, as of latest reports in 2011, still seemed to have a minor, sputtering presence of polio: Nigeria, India, Pakistan, Afghanistan, and China. The eradication campaign for poliomyelitis, unlike other well-meant and expensive global health initiatives, may succeed. Why? Because vaccinating humans by the millions is inexpensive, easy, and permanently effective, and because apart from infecting humans, the poliovirus has nowhere to hide. It’s not zoonotic.
Zoonotic pathogens can hide. That’s what makes them so interesting, so complicated, and so problematic.
Monkeypox is a disease similar to smallpox, caused by a virus closely related to variola. It’s a continuing threat to people in central and western Africa. Monkeypox differs from smallpox in one crucial way: the ability of its virus to infect nonhuman primates (hence the name) and some mammals of other sorts, including rats, mice, squirrels, rabbits, and American prairie dogs. Yellow fever, also infectious to both monkeys and humans, results from a virus that passes from victim to victim, and sometimes from monkey to human, in the bite of certain mosquitoes. This is a more complex situation. One result of the complexity is that yellow fever will probably continue to occur in humans—unless WHO kills every mosquito vector or every susceptible monkey in tropical Africa and South America. The Lyme disease agent, a type of bacterium, hides effectively in white-footed mice and other small mammals. These pathogens aren’t consciously hiding, of course. They reside where they do and transmit as they do because those happenstance options have worked for them in the past, yielding opportunities for survival and reproduction. By the cold Darwinian logic of natural selection, evolution codifies happenstance into strategy.
The least conspicuous strategy of all is to lurk within what’s called a reservoir host. A reservoir host (some scientists prefer “natural host”) is a living organism that carries the pathogen, harbors it chronically, while suffering little or no illness. When a disease seems to disappear between outbreaks (again, as Hendra did after 1994), its causative agent has got to be somewhere, yes? Well, maybe it vanished entirely from planet Earth—but probably not. Maybe it died off throughout the region and will only reappear when the winds and the fates bring it back from elsewhere. Or maybe it’s still lingering nearby, all around, within some reservoir host. A rodent? A bird? A butterfly? A bat? To reside undetected within a reservoir host is probably easiest wherever biological diversity is high and the ecosystem is relatively undisturbed. The converse is also true: Ecological disturbance causes diseases to emerge. Shake a tree, and things fall out.
Nearly all zoonotic diseases result from infection by one of six kinds of pathogen: viruses, bacteria, fungi, protists (a group of small, complex creatures such as amoebae, formerly but misleadingly known