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.

Shares of vaccine makers including Moderna, Novavax and CureVac rose Tuesday after U.S. health officials confirmed the first American death caused by the H5N1 bird flu. Moderna and Novavax shares rose more than 10%,

The first person to have a severe case of H5N1 bird flu in the United States has died, according to the Louisiana Department of Health.