In August 2004, a mysterious disease sickened chickens that a 32-year-old Thai woman was keeping at her home. She buried the last of her flock at the end of the month—wearing plastic bags on her hands for protection—and must have hoped that would be the end of it. It wasn’t: About a week later, the woman’s 11-year-old niece developed a high fever and severe respiratory symptoms; she died of pneumonia on September 8. Within days, the girl’s mother developed the same symptoms. She died on September 20.

By then, the chicken owner herself was gravely ill. And if the initial diagnosis of an upper respiratory infection had stuck, she likely would have died, too. But when she was admitted to a hospital, a team of doctors immediately quarantined her, started her on a course of the powerful antiviral drug Tamiflu—a treatment her relatives hadn’t received—and sent nose and throat swabs out for analysis to, among other places, the Centers for Disease Control and Prevention in Atlanta. The results confirmed that the woman had contracted something far worse than a chest cold. She had H5N1, an unusually deadly form of avian influenza.

Over the course of 2004, H5N1 was identified circulating through eight countries in Asia. Millions of chickens died from the disease, and millions more were slaughtered in a desperate attempt to contain it. Although there were only a handful of apparent cases of passing the virus between humans, the virus could sometimes infect people in close contact with affected birds—researchers theorized that’s what happened with the Thai girl, who used to play and sleep near her aunt’s chickens. And when the virus infected humans, it killed about 60 percent of them. That began “sparking fears that this lethal pathogen might cause a pandemic,” wrote the authors of a 2005 New England Journal of Medicine study about the Thai family.