With reports of the first human death from bird flu in the US, some Americans are feeling an uncomfortable flashback to the early days of Covid-19, when infectious disease experts were talking about a new virus that was sending people to the hospital with respiratory infections.

A ny time a person catches H5N1, or bird flu, their infection is a chance for the virus to mutate in the wrong direction. When someone dies from the bird flu—as an elderly Louisiana man did on Jan. 6, becoming the first U.S. death from the disease—experts get especially concerned.

Bird flu, also known as avian influenza or H5N1, has infected more than 130 million birds over the past two years as it threatened commercial bird operations and recently led to the first human death in the United States.