While the cause of Covid-19 was quickly identified as a coronavirus, DeRisi notes, that won’t necessarily be the case with whatever germ creates the next pandemic. And past strategies for detecting potentially dangerous viruses haven’t always been very systematic. “Different prevention projects in the past have just sort of picked up random roadkill on the side of the road and looked for viruses in it,” DeRisi told me. “Or they’ll look for all the viruses in bats.” While there’s a place for that sort of sampling, DeRisi said, it’s hard to know which of the many organisms discovered actually poses a risk. “Like, we have a project that’s examining the slurry in swine farms,” he went on. “And we’ve identified at least 200 novel viruses so far. Which is great! But we have no idea which of those, if any, have the ability to jump into humans — or how bad it would be if they did.” It would be better, DeRisi says, to watch for rare cases of mystery illnesses in people, which often exist well before a pathogen gains traction and is able to spread. Based on a retrospective analysis of blood samples, scientists now know that H.I.V. emerged nearly a dozen times over a century, starting in the 1920s, before it went global. Zika was a relatively harmless illness before a single mutation, in 2013, gave the virus the ability to enter and damage brain cells. Cristina Tato, an immunologist who runs the Biohub’s Rapid Response Team, points out that months before Zika exploded in Brazil, causing developmental issues and microcephaly in infants, researchers in the South Pacific noticed an increase in neurological symptoms, a missed clue that Zika was changing. “With pathogens, we’re much better at watching for things that we already know are out there,” DeRisi said. “Ebola, we know. Zika, we know. The beauty of this approach” — running blood samples from people hospitalized all over the world through his system, known as IDseq — “is that it works even for things that we’ve never seen before, or things that we might think we’ve seen but which are actually something new.”