A variant of the virus caused disease in people in 1997, when there was a small but deadly outbreak in Hong Kong. Eighteen people had confirmed diagnoses, and six of them died. Since then, there have been sporadic cases around the world—but no large outbreaks.

For influenza viruses, imperfection is a strength. They constantly mutate, producing new strains that challenge immune systems primed to fight earlier varieties. That’s what makes flu a life-long threat to humans and the animal species — mainly birds — that are vulnerable to it.