Infectious disease is all around us. Infectious disease is a kind of natural mortar binding one creature to another, one species to another, within the elaborate edifices we call ecosystems. It's one of the basic processes that ecologists study, including also predation, competition, and photosynthesis. Predators are relatively big beasts that eat their prey from outside. Pathogens (disease-causing agents, such as viruses) are relatively small beasts that eat their prey from within. Although infectious disease can seem grisly and dreadful, under ordinary conditions it's every bit as natural as what lions do to wildebeests, zebras, and gazelles.
But conditions aren't always ordinary.
Just as predators have their accustomed prey species, their favored targets, so do pathogens. And just as a lion might occasionally depart from its normal behavior—to kill a cow instead of a wildebeest, a human instead of a zebra—so can a pathogen shift to a new target. Accidents happen. Aberrations occur. Circumstances change and, with them, opportunities and exigencies also change. When a pathogen leaps from some nonhuman animal into a person, and succeeds there in making trouble, the result is what's known as a zoonosis.
The word zoonosis is unfamiliar to most people. But it helps clarify the biological reality behind the scary headlines about bird flu, SARS, other forms of nasty new disease, and the threat of a coming pandemic. It says something essential about the origin of HIV. It's a word of the future, destined for heavy use in the 21st century.
Ebola is a zoonosis. So is bubonic plague. So are yellow fever, monkeypox, bovine tuberculosis, Lyme disease, West Nile fever, Marburg, many strains of influenza, rabies, hantavirus pulmonary syndrome, and a strange new affliction called Nipah, which kills pigs and pig farmers in Malaysia. Each of them reflects the action of a pathogen that can cross to people from other species. This form of interspecies leap is common, not rare; about 60 percent of all human infectious diseases currently known are shared between animals and humans. Some of those—notably rabies—are widespread and famously lethal, still killing humans by the thousands despite centuries of effort at coping with their effects, concerted international attempts to eradicate or control them, and a clear scientific understanding of how they work. Others are new and inexplicably sporadic, claiming a few victims (as Hendra did) or a few hundred in this place or that, and then disappearing for years.
Smallpox, to take one counterexample, is not a zoonosis. It's caused by a virus that infects Homo sapiens and, in very exceptional cases, certain nonhuman primates, but not horses or rats or other species. That helps explain why the World Health Organization's global campaign to eradicate the disease was, as of 1979, successful. Smallpox could be eradicated because its virus, lacking ability to reside virtually anywhere other than in humans, couldn't hide. Zoonotic pathogens can hide.