Monkeypox, though closely related to smallpox, differs in two crucial ways—its propensity to afflict monkeys as well as humans, and the ability of its virus to exist in still other species, some of which are so far unidentified. Yellow fever, also infectious to both monkeys and humans, and caused by a virus that hides in several species of mosquito, will probably never be eradicated. The Lyme disease perpetrator, a type of bacterium, hides effectively in white-footed mice and other small mammals. These pathogens aren't consciously hiding, of course. For their purposes, such behavior merely constitutes a strategy of indirect transmission or inconspicuous survival.
The least conspicuous strategy of all is to lurk within what's called a reservoir host, a species that carries the pathogen while suffering little or no symptomatic illness. When a disease seems to disappear between outbreaks (again, as Hendra did after the 1994 carnage), its causal pathogen may indeed have died out, at least from the region—but then again, maybe not. Maybe its still lingering nearby, all around, within some reservoir host. A rodent? A bird? A butterfly? Possibly 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.
Some months after the deaths in Australia, a scientific sleuth named Hume Field started looking for Hendra's reservoir host. Field was a veterinarian who, having practiced privately for years, had decided to pursue a doctorate in veterinary epidemiology. The search for the reservoir became his dissertation project. He gathered blood samples from 16 different species, a whole menagerie of suspects, including marsupials, birds, rodents, amphibians, and insects. He sent the samples to a laboratory for screening, which yielded no evidence whatsoever of Hendra.
Then he took blood from Pteropus alecto, a species of fruit bat, big as a crow and commonly known as the black flying fox. Bingo: The lab team found molecular traces left by Hendra virus. Further sampling produced similar evidence from three other species of flying foxes, all native to the forests of Queensland (the state encompassing Brisbane) and other wooded regions of Australia. Field and his collaborators had established that bats were the reservoir. Detecting molecular traces is less definitive than finding particles of live virus, but within one female bat they did find that form of evidence also.
The lab work suggested that Hendra was an old virus, having probably existed within its reservoir host for thousands of years. Despite its age, it had never before—so far as historical records and human memory could say, anyway—caused disease in humans. What accounts for its emergence in 1994? Well, bad luck for Drama Series and those who knew her. Bats came to eat the figs in that solitary tree, and the poor mare, seeking shade, grazing too carelessly, evidently swallowed not just grass but also something of what they dropped, such as fruit pulp, feces, urine, afterbirth, and virus.