Spillover - By David Quammen Page 0,3

kidney during his autopsy. That sample also yielded a virus, identical to the virus from the horses, confirming that this equine morbillivirus didn’t afflict only equines. Later, when the degree of its uniqueness became better appreciated, the label “EMV” was dropped and the virus was renamed after its place of emergence: Hendra.

Identifying the new virus was only step one in solving the immediate mystery of Hendra, let alone understanding the disease in a wider context. Step two would involve tracking that virus to its hiding place. Where did it exist when it wasn’t killing horses and people? Step three would entail asking a further cluster of questions: How did the virus emerge from its secret refuge, and why here, and why now?

After our first conversation, at a café in Hendra, Peter Reid drove me several miles southeast, across the Brisbane River, to the site where Drama Series took sick. It was in an area called Cannon Hill, formerly pastoral land surrounded by city, now a booming suburb just off the M1 motorway. Tract houses on prim lanes had been built over the original paddock. Not much of the old landscape remained. But toward the end of one street was a circle, called Calliope Circuit, in the middle of which stood a single mature tree, a Moreton Bay fig, beneath which the mare would have found shelter from eastern Australia’s fierce subtropical sun.

“That’s it,” Reid said. “That’s the bloody tree.” That’s where the bats gathered, he meant.

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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 biophysical edifices we call ecosystems. It’s one of the basic processes that ecologists study, including also predation, competition, decomposition, 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 and zebras, or what owls do to mice.

But conditions aren’t always ordinary.

Just as predators have their accustomed prey, 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, exigencies and opportunities change too. When a pathogen leaps from some nonhuman animal into a person, and succeeds there in establishing itself as an infectious presence, sometimes causing illness or death, the result is a zoonosis.

It’s a mildly technical term, zoonosis, unfamiliar to most people, but it helps clarify the biological complexities behind the ominous headlines about swine flu, bird flu, SARS, emerging diseases in general, and the threat of a global pandemic. It helps us comprehend why medical science and public health campaigns have been able to conquer some horrific diseases, such as smallpox and polio, but unable to conquer other horrific diseases, such as dengue and yellow fever. It says something essential about the origins of AIDS. It’s a word of the future, destined for heavy use in the twenty-first century.

Ebola is a zoonosis. So is bubonic plague. So was the so-called Spanish influenza of 1918–1919, which had its ultimate source in a wild aquatic bird and, after passing through some combination of domesticated animals (a duck in southern China, a sow in Iowa?) emerged to kill as many as 50 million people before receding into obscurity. All of the human influenzas are zoonoses. So are monkeypox, bovine tuberculosis, Lyme disease, West Nile fever, Marburg virus disease, rabies, hantavirus pulmonary syndrome, anthrax, Lassa fever, Rift Valley fever, ocular larva migrans, scrub typhus, Bolivian hemorrhagic fever, Kyasanur forest disease, and a strange new affliction called Nipah encephalitis, which has killed pigs and pig farmers in Malaysia. Each of them reflects the action of a pathogen that can cross into people from other animals. AIDS is a disease of zoonotic origin caused by a virus that, having reached humans through just a few accidental events in western and central Africa, now passes human-to-human by the millions. This form of interspecies leap is common, not rare; about 60 percent of all human infectious diseases currently known either cross routinely or have recently crossed between other animals and us. Some of those—notably rabies—are familiar, widespread, and still horrendously lethal, killing humans by the thousands despite