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It was late one night in January 2009, and Jonathan Epstein was standing on the roof of an abandoned storage depot near Khulna, Bangladesh, with the writer and journalist David Quammen along with a small team of veterinarians. The group was in Bangladesh on a strange errand: They were catching bats.

It had been more than a decade since the first outbreak of the Nipah virus in Malaysia. Nipah, named after the home village of one of its earliest victims, causes respiratory distress, inflammation of the brain, and seizures. Its mortality rate is staggeringly high — between 40 and 75 percent of those who contract the disease ultimately die. (The virus depicted in the now all-too-prescient 2011 film Contagion was based on Nipah.)

In Malaysia, the initial outbreak of Nipah in 1998 had infected 283 people and killed 109. Scientists eventually discovered that the virus had been passed from bats to local pig farms; the government slaughtered over 1 million pigs in an effort to stop the spread. But Nipah kept coming back, popping up in new places around the world, killing hundreds. Epstein and his colleagues were in that storage depot in Bangladesh trying to understand if the bats there were also carriers of the virus — and if they might pass it to humans again.

Epstein, a veterinarian and disease ecologist at the nonprofit EcoHealth Alliance in New York City, has been all over the world identifying and tracing viruses that could make a deadly jump from animals to humans, helped by sprawling cities, clear-cut forests, and other human encroachment into the natural world. He helped trace the first outbreak of Severe Acute Respiratory Syndrome, or SARS, in 2003 to horseshoe bats in China; the outbreak of Middle East Respiratory Syndrome, or MERS, in 2013 to camels and bats; and — when it’s safe to travel again — he will return to China to help pinpoint the source of the current coronavirus, SARS-CoV-2. Bats are, once again, the main suspects.

“There are always two pivotal questions,” Epstein said by phone from his home in Queens. “How did this happen? And could it happen again?”

Sixty percent of new infectious diseases — diseases that, like COVID-19, have never before reached humans — originate in domesticated animals and wildlife, often bats, rodents, or non-human primates. Scientists estimate that there are as many as 800,000 of these so-called zoonotic viruses lurking in the natural world that could infect humans. The animals carrying these viruses often don’t get sick; instead, they serve as “reservoirs,” amassing pathogens as they eat, sleep, and socialize. It’s a good deal for the viruses: They get a free ride, while they wait for a chance to make a cross-species leap.

The problem is that those deadly leaps are becoming more common. Population growth and environmental and habitat destruction are bringing humans into more frequent contact with certain species — and the viruses that they carry.

“Every future viral threat that people could get already exists and is circulating in those animals — always has been,” said Dennis Carroll, an expert on zoonotic infectious diseases and the former head of the emerging threats division of the United States Agency for International Development, or USAID. “It’s just that now we’re bumping into them with a frequency that enables spillover to occur.”

As COVID-19 spreads across the globe, infecting almost 3 million people worldwide and killing over 200,000, public health officials, policymakers, and journalists are analyzing what went wrong. Why was the world so unprepared? Should countries have higher national stockpiles of ventilators, masks, ICU beds? Should stay-at-home orders have been issued earlier?

These are all measures for after an epidemic has begun, after an enterprising virus has leapt from an animal to a human (a phenomenon scientists call “spillover”). But a small group of ecologists, epidemiologists, and veterinarians have spent the last decade attacking pandemics from another angle. Epstein and other researchers have been out in the field, testing bats in Bangladesh and pigs in West Africa, trying to catalog a huge number of viruses in the hopes of preventing a spillover into human populations. Because there’s no outbreak if the virus never makes the jump.