In 1981, a young man visited Cedars-Sinai hospital, in Los Angeles, with shortness of breath and with curious purplish lesions on his skin. After reviewing biopsies and scans, a twenty-eight-year-old medical resident named David Ho found an odd fungal infection in the patient’s lungs and a rare cancer, Kaposi’s sarcoma. These conditions were both associated with immune deficiency, though nothing in the patient’s history explained why he would be in such a state. He was given antibiotics and discharged; not long after, he died. Over a few months, Ho and his colleagues saw five men with similar symptoms. They wrote up the cases and sent them to the Centers for Disease Control—the first report of what became known as AIDS.

Ho continued to explore the disease. “Some people were very concerned that I was so intrigued by those few cases at the very beginning of my career,” he told me. “ ‘Why would you want to devote your career to an esoteric disease?’ ” Particularly one that seemed mainly to afflict what was considered a fringe population—gay men. But Ho, who had emigrated from Taiwan when he was twelve, speaking no English, had an underdog mentality and would not be dissuaded.

He made several discoveries throughout the nineteen-eighties about H.I.V., the virus that causes AIDS, and in 1990, at the age of thirty-seven, he moved to New York to become the director of the Aaron Diamond AIDS Research Center. A year later, he received a call asking him to fly back to L.A. to test a very important patient. There, he confirmed that Earvin (Magic) Johnson was H.I.V.-positive. The following week, Johnson disclosed his condition and announced that he was retiring from the N.B.A. Ho has cared for him ever since. Johnson later said that he’d never thought AIDS would kill him, because Ho had assured him that better medicines were in the pipeline. In 1994, Ho found that a certain class of drugs could dramatically reduce the viral load in AIDS patients. But, within each infected individual, the virus evolved quickly, evading treatments. One drug was not enough. His team devised the idea of an AIDS “cocktail”—a combination of three or four drugs that, acting in concert, could corner the virus. In 1996, Time named Ho its Man of the Year.

In November, 2002, a novel disease broke out in China: severe acute respiratory syndrome, caused by a coronavirus called SARS-CoV. Ho was asked by China’s top public-health officials to advise them. “The most dramatic memory I have is going to Beijing, arriving in the late afternoon or early evening, and going to the hotel along the biggest avenue,” he recalled. “If you remember the Tiananmen incidents of many years ago, with the protester and the tank, that’s the boulevard. It has ten or twelve lanes. There was only the car that’s driving me and one ambulance for as far as one could see.”

He went on, “That’s when I got interested in coronaviruses, serving as a consultant and seeing the devastation firsthand in several cities throughout China.” Back in New York, Ho began investigating the coronavirus family. Some coronaviruses can produce lethal diseases, like SARS; others are among the causes of the common cold. But, he said, “the SARS epidemic ended in July of 2003. By the next year, there was hardly any interest. Funding for that area kind of dried up. So we simply dropped it and went on with our H.I.V. work.” In 2012, another coronavirus, MERS-CoV, caused an outbreak in the Arabian Peninsula; Middle East respiratory syndrome, as it was called, sickened more than twenty-five hundred people and killed more than eight hundred. Ho followed it with interest, but this outbreak, too, passed quickly. Then, this past December, a disease with similar symptoms flared up in China and, within a month, was linked to another coronavirus, SARS-CoV-2. Ho told me, “My Chinese heritage caused me to focus more on the news coming out of China in late December and early January. However, the experience with SARS also put a pause on our natural reaction to jump in and get involved.” His attitude shifted when the story did. “It was the growing magnitude of the outbreak that told us, ‘Oh, we’d better think about getting into this,’ ” he said.

Ho was just setting up his lab at its new home, at Columbia University. He is friendly with Jack Ma, the founder of the e-commerce giant Alibaba, who asked how he could help. In February, Columbia announced that Ma’s foundation had awarded a $2.1-million grant to Ho and several Columbia colleagues to develop antiviral drugs. This project was prompted by the COVID-19 crisis, but the mission goes beyond it; the researchers are thinking not only about the current pandemic but about future ones as well.

What will the next global pathogen be? “If you’d asked me that three or four months ago, I would have said influenza,” Ho told me, with a chuckle of dismay. For scientists, this isn’t just a thought experiment; it’s the sort of question that shapes years of research. Two years ago, a team at Johns Hopkins issued a report titled “The Characteristics of Pandemic Pathogens,” which was based on a literature review, interviews with more than a hundred and twenty experts, and a meeting devoted to the issue. It grimly considered the possibilities.

Could bacteria do us in? Outbreaks of plague have wreaked havoc throughout history, but the development of effective antibiotics in the past century “took bacteria off the table as a global biological risk for the most part,” Amesh Adalja, a physician at Johns Hopkins and the report’s project director, told me. Bacteria can evolve, and develop drug resistance, but usually not quickly. How about fungi? They threaten some species, but don’t adapt well to warm-blooded hosts (and may have helped encourage the evolution of warm-bloodedness). Prions? These are responsible for mad-cow disease and its human variant, but are mostly avoidable by preventing food contamination and refraining from cannibalism. Protozoa? Malaria has killed perhaps half of all humans who have ever lived. But protozoa are typically transmitted by vectors such as mosquitoes and fleas, which are limited by climate and geography. Viruses, the report concluded, are the real menaces.

Not just any viruses, though. The likeliest candidates are those with a genome of RNA, which evolve faster than those with DNA. Viruses that spread before symptoms appear also have a considerable advantage. (The only infectious disease we’ve wiped out, smallpox, is not contagious during the incubation period.) And the most daunting are those transmitted by respiration, rather than by feces or bodily fluids, which can be controlled through sanitation. Viruses that can move between animals and humans are especially hard to manage. All in all, this character sketch gets us pretty close to identifying two classes of viral assailants: influenzas and coronaviruses.

None of our off-the-shelf treatments equip us for such a pandemic. If bacteria invade, there’s a long list of antibiotics you can try. Between ciprofloxacin and amoxicillin, we can treat dozens of different types of bacterial infection. For the roughly two hundred identified viruses that afflict us, there are approved treatments for only ten or so. And the antiviral drugs that exist tend to have narrow targets. Only a few have been approved for use against more than one disease. Many drugs that work on one virus don’t work on others within the same family; antivirals suited for some herpesviruses (such as the one that causes chicken pox and shingles) aren’t suited for others. Some antivirals can’t even treat different strains of the same virus.