Spillover: Animal Infections and the Next Human Pandemic

by David Quammen

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.

“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.

“If you look at the world from the point of view of a hungry virus,” the historian William H. McNeill has noted, “or even a bacterium—we offer a magnificent feeding ground with all our billions of human bodies, where, in the very recent past, there were only half as many people. In some 25 or 27 years, we have doubled in number. A marvelous target for any organism that can adapt itself to invading us.”

Just in from the airport, Chmura and I joined a group of his friends at Sun Yat-sen University and plunged into a snack of the world’s stinkiest fruit. It’s a large spiky thing, a durian, like a puffer fish that has swallowed a football; pried open, it yields individual gobbets of glutinous creamy pulp, maybe eight or ten gobbets per fruit, and an unwelcoming bouquet. The pulp tastes like vanilla custard and smells like the underwear of someone you don’t want to know. We ate barehanded, slurping the goo between our fingers as it oozed and dripped. This was before dinner, in lieu of peanuts and beer. Then we went out to a restaurant where Chmura ordered us a dish featuring congealed pig’s blood—in little hepatic cubes, like diced liver—with bean sprouts and hot red peppers. By late evening my shirt was soaked with sweat. Welcome to China. But I was keen to learn what Aleksei Chmura knew, to benefit from his voracious curiosity, and I would eat my way toward insight at his side, if necessary.

“The parasitic mode of life is essentially similar to that of the predatory carnivores. It is just another method of obtaining food from the tissues of living animals,” though with parasites the consumption tends to be slower and more internalized within the prey. Small creatures eat bigger ones, generally from the inside out.

White-tailed deer seem to play a much different role. They are important mainly to adult ticks—not just for their blood, but also for providing a venue where male blacklegged ticks can meet females. A whitetail in the woods of Connecticut, during November, is like a teeming singles’ bar in lower Manhattan on Friday night, crowded with lubricious seekers. One poor doe might be carrying a thousand mature blacklegged ticks. Mating occurs, somewhat gracelessly, when a male tick, prowling across the skin of the deer, encounters a preoccupied female—she is tapped in, drinking, immobile. Don’t look for romance in arachnoid sex.

certain points about Lyme disease emerge plainly. “We know that walking into a small woodlot,” he wrote, “is riskier than walking into a nearby large, extensive forest. We know that hiking in the oak woods two summers after a big acorn year is much riskier than hiking in those same woods after an acorn failure. We know that forests that house many kinds of mammals and birds are safer than those that support fewer kinds. We know that the more opossums and squirrels there are in the woods, the lower the risk of Lyme disease, and we suspect that the same is true of owls, hawks, and weasels.” As for white-tailed deer: They’re involved, yes, but far from paramount, so don’t believe everything you’ve heard.

On our first afternoon in Sylhet, we scouted a holy place known as Chashnipeer Majar. It’s a small domed structure atop a hillock that looms above a crowded neighborhood, surrounded below by concrete walls, small shops, blank-faced houses fronting the street, and sinuous alleys. A long staircase led us to the shrine, which was overarched by five or six scraggly trees, one with dead limbs where monkeys perched, shaking the branches like mad sailors in a ship’s rigging. The hillsides around the shrine were covered with ragged brush, trash, and the graves of Sylhetian ancestors. It wasn’t a verdant spot, this little island of sacred ground at the heart of an urban neighborhood, but the resident wildlife didn’t seem to mind. There were macaques on the shrine roof, macaques in the trees, macaques on rooftops of the houses below, macaques climbing drainpipes, macaques crossing power lines, macaques loitering on the staircase and walking its railings, macaques scampering among the graves.

My assignment was minor: When the door fell, I should get there immediately and lock it down with my foot, so the captured macaques couldn’t widget their way out. Gregory Engel would then do the hard part, tranquilizing them one by one with a hypodermic full of Telazol, a fast-acting veterinary anesthetic. How do you inject a hysterical monkey? In this case, by jabbing into its thigh through the mesh of the trap. Professor Mohammed Mustafa Feeroz, Engel and Jones-Engel’s principal Bangladeshi collaborator, would stand as defense. Four of Feeroz’s students would help. Defense was important because the uncaptured monkeys might charge, frantic to free their comrades. They could be a formidable platoon. Lisa Jones-Engel, chief genius of the whole project but prohibited from entering this shrine because of her gender, would be waiting in a courtyard nearby, along with several female assistants, to begin drawing blood. One, two, three: trap, tranquilize, draw. What could be simpler? Lots of things, let me tell you, could be simpler. The trap was baited with puffed rice and bananas. Within moments of seeing the bait placed, a few monkeys came to inspect. They climbed all over the trap, inside and out. Most of the others held back. Word seemed to pass among them, excitement rose, more animals arrived across the rooftops; there must have been a hundred, all nervously curious about our presence and tantalized by the bait. We loitered discreetly, on the steps, on the slope, looking c...

Lisa Jones-Engel is a forceful, direct person with years of experience among Asia’s nonhuman primates. She loves her subject animals but doesn’t romanticize them. As she and her assistants started drawing blood and taking oral swabs, her husband and Feeroz, followed by the male students and me, headed back to the shrine for another round of trapping. Now that we had shown our methods, and our devious intentions, it was dicey to say how the troop might behave. “If the monkeys in the last half hour have figured out their plan of attack,” Lisa commanded us, “you just retreat.”

Within a decade, though, two things happened: The virus became inherently less virulent and the surviving rabbits became more resistant to it. Mortality fell and the rabbit population began climbing back. This is the short, simple version of the story, with its facile lesson: Evolution lowers virulence, tending toward that “more perfect mutual tolerance” between pathogen and host. Well, not quite. The real story, teased out through careful experimental work by an Australian microbiologist named Frank Fenner and his colleagues, is that virulence declined quickly from its original extreme, north of 99 percent, and then stabilized at a lower level that was still pretty damn high. Would you consider a kill rate of “just” 90 percent to be mutual tolerance? Me neither. That’s as lethal as Ebola virus, at its most extreme, in a Congolese village. But it’s what Fenner found.

The first rule of a successful parasite? Myxoma’s success in Australia suggests something different from that nugget of conventional wisdom I mentioned above. It’s not Don’t kill your host. It’s Don’t burn your bridges until after you’ve crossed them.

They were magnificent. I had seen fruit bats in Asia before, but never so many in motion so close. I must have been gawking like a fool because Epstein gently advised, “Keep your mouth closed when you look up.” They shed Nipah virus in their urine, he reminded me.

Are you vaccinated for rabies, David? (Yes.) When was your last booster, how are your titers? (Um, don’t know.) As for Nipah exposure, never mind, because there’s no vaccine, no treatment, no cure. (What a relief.) Have I said, Don’t get bit? Our first principles are, one, safety for us; two, safety for the bats. Let’s do take good care of the bats, Epstein said. (He’s a veterinarian and a conservationist, before all.) Any questions?

An American field biologist once expressed to me how he felt in such moments. “That’s the name of the game: getting home with the data.” This man’s research too involved dangers—shipwreck, starvation, drowning, snakebite, though not malaria and AK rifles. “If you take too many risks, you don’t get home,” he said. “If you take too few, you don’t get the data.”

That’s the salubrious thing about zoonotic diseases: They remind us, as St. Francis did, that we humans are inseparable from the natural world. In fact, there is no “natural world,” it’s a bad and artificial phrase. There is only the world. Humankind is part of that world, as are the ebolaviruses, as are the influenzas and the HIVs, as are Nipah and Hendra and SARS, as are chimpanzees and bats and palm civets and bar-headed geese, as is the next murderous virus—the one we haven’t yet detected.