But there had to be a broader answer, too. Why did Hendra emerge in 1994, not decades or centuries earlier? Something was different. Some sort of change, or combination of changes, must have caused the virus to be transferred from its reservoir host into other species.
The fancy name for such transfer is spillover. Maybe the virus needed horses (which only reached Australia with European colonists), as distinct from kangaroos (which have been eating grass beneath Australian fig trees for millennia), to mediate its spillover from the reservoir. Maybe bats, figs, horses, and humans had simply never been brought so closely together. Hume Field is currently a research scientist at the Animal Research Institute of Queensland's Department of Primary Industries, in Brisbane. When I spoke with him at his office there, he raised the issue of "what might be happening now that hasn't happened before." Part of the answer is that human destruction of eucalyptus forests has disrupted the customary feeding and roosting habits of some flying foxes, forcing them toward shady suburbs, orchards, botanical gardens, city parks, and closer proximity to people.
But proximity is one thing; spilling virus into horses is another. "How does transmission occur?" Field wondered aloud, at the end of our long conversation. "Well, we still don't know."
Nearly all zoonotic diseases result from infection by one of six kinds of pathogen: viruses, bacteria, protozoans, prions, fungi, and worms. Mad cow disease is caused by a prion, a weirdly folded protein molecule that triggers weird folding in other molecules, like Kurt Vonnegut's infectious form of water, ice-nine, in his great early novel Cat's Cradle. Sleeping sickness is a protozoan infection, carried by tsetse flies between wild and domestic mammals and people on the landscapes of sub-Saharan Africa. Anthrax is a bacterium that can live dormant in soil for years and then, when scuffed out, infect humans by way of cattle. Toxocariasis is a mild zoonosis caused by roundworms; you can get it from your dog. But fortunately, like your dog, you can be wormed.
Viruses are the most problematic. They evolve quickly, they are unaffected by antibiotics, they can be elusive, they can be versatile, they can inflict extremely high rates of mortality, and they are fiendishly simple, at least relative to other living or quasi-living creatures. Hanta, SARS, monkeypox, rabies, Ebola, West Nile, Machupo, dengue, yellow fever, Junin, Nipah, Hendra, influenza, and HIV are all viruses. The full list is much longer. There is a thing known by the vivid name simian foamy virus (SFV) that infects monkeys and humans in Asia by way of venues (such as Buddhist and Hindu temples) where people and half-tame macaques come into close contact. Some of the people visiting those temples, feeding handouts to those macaques, exposing themselves to SFV, are international tourists. "Viruses have no locomotion," according to the eminent virologist Stephen S. Morse, "yet many of them have traveled around the world." They can't run, they can't walk, they can't swim, they can't crawl. They ride.