David Quammen writes about emerging infectious diseases in National Geographic (Oct 2007):

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.

…

Close contact between humans and other species can occur in various ways: through killing and eating of wild animals (as in Mayibout II), through caregiving to domestic animals (as in Hendra), through fondling of pets (as with monkeypox, brought into the American pet trade by way of imported African rodents), through taming enticements (feeding bananas to the monkeys at a Balinese temple), through intensive animal husbandry combined with habitat destruction (as on Malaysian pig farms), and through any other sort of disruptive penetration of humans into wild landscape—of which, needless to say, there’s plenty happening around the world. Once the contact has occurred and the pathogen has crossed over, two other factors contribute to the possibility of cataclysmic consequences: the sheer abundance of humans on Earth, all available for infection, and the speed of our travel from one place to another. When a bad new disease catches hold, one that manages to be transmissible from person to person by a handshake, a kiss, or a sneeze, it might easily circle the world and kill millions of people before medical science can find a way to control it.

But our safety, our health, isn’t the only issue. Another thing worth remembering is that disease can go both ways: from humans to other species as well as from them to us. Measles, polio, scabies, influenza, tuberculosis, and other human diseases are considered threats to non-human primates. The label for those infections is anthropozoonotic. Any of them might be carried by a tourist, a researcher, or a local person, with potentially devastating impacts on a tiny, isolated population of great apes with a relatively small gene pool, such as the mountain gorillas of Rwanda or the chimps of Gombe.