it wouldn’t crawl up his face, letting it free its legs and its wings, then relaxing his grip gently just as the wing beats began to find purchase on air, and watching—all of us watching—the animal catch itself short of the ground, rise slowly, circle languidly, and fly away. Eventually, after a circuit or two of the compound, a few minutes of befuddled relief, it would find its way back to the communal roost, sadder but wiser and no great harm done.
Before releasing the last bat, Epstein gave a brief talk to the assembled citizens, translated by Arif, congratulating them on their good fortune as a village at harboring so many wonderful bats, which are helpful to fruit trees and other plants, and assuring them that he and his colleagues had taken great care not to harm the animals while studying their health. Then he let the final bat drop. It climbed through the air, from knee level, and flew away.
Later he said to me: “Any one of those six bats could have been infected. That’s what it looks like. They look totally healthy. There’s no way to distinguish Nipah virus. That’s why we take all these precautions.” He dipped his boots again in the sterile footbath, as we left the lab, and washed up at the village pump. A little girl brought soap.
75
“The key is connectivity,” Epstein told me, during a quiet chat the following afternoon. “The key is to understand how animals and people are interconnected.” We were back at the hotel, showered and fed, after another full night of trapping, another fifteen bats sampled and released. You can’t look at a new bug or a reservoir host, he said, as though they exist in a vacuum. It’s a matter of contact with humans, interaction, opportunity. “Therein lies the risk of spillover.”
Repeatedly over the next half hour he returned to the word “opportunity.” It kept knocking. “A lot of these viruses, a lot of these pathogens that come out of wildlife into domestic animals or people, have existed in wild animals for a very long time,” he said. They don’t necessarily cause any disease. They have coevolved with their natural hosts over millions of years. They have reached some sort of accommodation, replicating slowly but steadily, passing unobtrusively through the host population, enjoying long-term security—and eschewing short-term success in the form of maximal replication within each host individual. It’s a strategy that works. But when we humans disturb the accommodation—when we encroach upon the host populations, hunting them for meat, dragging or pushing them out of their ecosystems, disrupting or destroying those ecosystems—our action increases the level of risk. “It increases the opportunity for these pathogens to jump from their natural host into a new host,” he said. The new host might be any animal (the horse in Australia, the palm civet in China) but often it’s humans, because we are present so intrusively and abundantly. We offer a wealth of opportunity.
“Sometimes nothing happens,” Epstein said. A leap is made but the microbe remains benign in its new host, as it was in the old one. (Simian foamy virus?) In other cases, the result is very severe disease for a limited number of people, after which the pathogen comes to a dead end. (Hendra, Ebola.) In still other cases, the pathogen achieves great and far-reaching success in its new host. It finds itself well enough suited to get a foothold; it makes itself still better suited by adapting. It evolves, it flourishes, it continues. The history of HIV is the story a leaping virus that might have come to a dead end but didn’t.
Yes, HIV is a vivid example, I agreed. But is there any particular reason why other RNA viruses shouldn’t have the same potential? For instance, Nipah?
“No reason at all. There’s no reason at all,” Epstein said. “A lot of what determines whether a pathogen becomes successful in a new host, I think, is odds. Chance, to a large degree.” With their high rates of mutation, their high rates of replication, RNA viruses are very adaptable, he reminded me, and every spillover presents a new opportunity to adapt and take hold. We’ll probably never know how often that occurs—how many animal viruses spill into people inconspicuously. Many of those viruses cause no disease, or they cause a new disease that—in some parts of the world, because health care is marginal—gets mistaken for an old disease. “The point being,” he said, “that the more opportunity viruses have