For thousands of years, a parasite with no name lived happily among horseshoe bats in southern China. The bats had evolved to the point that they did not notice; they went about their nightly flights unbothered. One day, the parasite—an ancestor of the coronavirus, SARS-CoV-2—had an opportunity to expand its realm. Perhaps it was a pangolin, the scaly anteater, an endangered species that is a victim of incessant wildlife trafficking and sold, often secretly, in live-animal markets throughout Southeast Asia and China. Or not. The genetic pathway remains unclear. But to survive in a new species, whatever it was, the virus had to mutate dramatically. It might even have taken a segment of a different coronavirus strain that already inhabited its new host, and morphed into a hybrid—a better, stronger version of itself, a pathogenic Everyman capable of thriving in diverse species. More recently, the coronavirus found a new species: ours. Perhaps a weary traveller rubbed his eyes, or scratched his nose, or was anxiously, unconsciously, biting his fingernails. One tiny, invisible blob of virus. One human face. And here we are, battling a global pandemic.

The world’s confirmed cases (those with a positive lab test for COVID-19, the disease caused by SARS-CoV-2) doubled in seven days, from nearly two hundred and thirteen thousand, on March 19th, to four hundred and sixty-seven thousand, on March 26th. Nearly twenty-one thousand people have died. The United States now has more confirmed cases than any country on earth, with more than eighty thousand on March 26th. These numbers are a fraction of the real, unknown total in this country and around the world, and the numbers will keep going up. Scientists behind a new study, published earlier this month in the journal Science, have found that for every confirmed case there are likely five to ten more people in the community with an undetected infection. This will likely remain the case. “The testing is not near adequate,” one of the study’s authors, Jeffrey Shaman, an environmental-health sciences professor at Columbia University, said. Comments from emergency-room doctors have been circulating on social media like S.O.S. flares. One, from Daniele Macchini, a doctor in Bergamo, north of Milan, described the situation as a “tsunami that has overwhelmed us.”

Scientists first discovered that coronaviruses originate among bats following the outbreak of Severe Acute Respiratory Syndrome (SARS) in 2003. Jonathan Epstein, an epidemiologist at the EcoHealth Alliance in New York who studies zoonotic viruses—those that can jump from animals to people—was part of a research team that went hunting for the source in China’s Guangdong Province, where simultaneous SARS outbreaks had occurred, suggesting multiple spillovers from animals to people. At first, health officials believed palm civets, a mongoose-like species commonly eaten in parts of China, were responsible, as they were widely sold at markets connected to the SARS outbreak, and tested positive for the virus. But civets bred elsewhere in Guangdong had no antibodies for the virus, indicating that the market animals were only an intermediary, highly infectious host. Epstein and others suspected that bats, which are ubiquitous in the area’s rural, agricultural hills, and were, at the time, also sold from cages at Guangdong’s wet markets, might be the coronavirus’s natural reservoir.

The researchers travelled through the countryside, setting up field labs inside limestone caverns and taking swabs from dozens of bats through the night. After months of investigation, Epstein’s team discovered four species of horseshoe bats that carried coronaviruses similar to SARS, one of which carried a coronavirus that was, genetically, a more than ninety per cent match. “They were found in all of the locations where SARS clusters were happening,” he said.

After years of further bat surveillance, researchers eventually found the direct coronavirus antecedent to SARS, as well as hundreds of other coronaviruses circulating among some of the fourteen hundred bats species that live on six continents. Coronaviruses, and other virus families, it turns out, have been co-evolving with bats for the entire span of human civilization, and possibly much longer. As the coronavirus family grows, different strains simultaneously co-infect individual bats, turning their little bodies into virus blenders, creating new strains of every sort, some more powerful than others. This process happens without making bats sick—a phenomenon that scientists have linked to bats’ singular ability, among mammals, to fly. The feat takes a severe toll, such that their immune systems have evolved a better way to repair cell damage and to fight off viruses without provoking further inflammation. But when these viruses leap into a new species—whether a pangolin or a civet or a human—the result can be severe, sometimes deadly, sickness.

In 2013, Epstein’s main collaborator in China, Shi Zheng-Li, sequenced a coronavirus found in bats, which, in January, she discovered shares ninety-six per cent of its genome with SARS-CoV-2. The two viruses have a common ancestor that dates back thirty to fifty years, but the absence of a perfect match suggests that further mutation took place in other bat colonies, and then in an intermediate host. When forty-one severe cases of pneumonia were first announced in Wuhan, in December, many of them were connected to a wet market with a notorious wildlife section. Animals are stacked in cages—rabbits on top of civets on top of ferret-badgers. “That’s just a gravitational exchange of fecal matter and viruses,” Epstein said. Chinese authorities reported that they tested animals at the market—all of which came back negative—but they have not specified which animals they tested, information that is crucial for Epstein’s detective work. Authorities later found the virus in samples taken from the market’s tables and gutters. But, because not all of the first patients were tied to the market, nor were they connected to one another, Epstein said, “it raised the question of, well, perhaps those forty-one weren’t the first cases.”

Analyses of the SARS-CoV-2 genome indicate a single spillover event, meaning the virus jumped only once from an animal to a person, which makes it likely that the virus was circulating among people before December. Unless more information about the animals at the Wuhan market is released, the transmission chain may never be clear. There are, however, numerous possibilities. A bat hunter or a wildlife trafficker might have brought the virus to the market. Pangolins happen to carry a coronavirus, which they might have picked up from bats years ago, and which is, in one crucial part of its genome, virtually identical to SARS-CoV-2. But no one has yet found evidence that pangolins were at the Wuhan market, or even that venders there trafficked pangolins. “We’ve created circumstances in our world somehow that allows for these viruses, which would otherwise not be known to cause any problems, to get into human populations,” Mark Denison, the director of pediatric infectious diseases at Vanderbilt University Medical Center’s Institute for Infection, Immunology, and Inflammation, told me. “And this one happened to say, ‘I really like it here.’ ”