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. Although both viruses can cause breathing problems, they are very different.

Covid was spreading easily from person to person when it arrived in the US in 2020, but bird flu has been lurking for years, mostly as a problem for animals. Scientists also know a lot more about H5N1 bird flu than they did the SARS-CoV-2 virus, and the US has been preparing for the threat of a new flu outbreak for a long time.

Still, the virus is making moves that deserve attention. Here’s what you need to know about H5N1.