Human bird flu cases are increasing across the country. The H5N1 virus primarily infects poultry and mammals but can be transmitted to humans via direct contact with sick animals, per the Centers for Disease Control and Prevention (CDC).

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.

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.

The CDC says the risk of bird flu to the general public is low, but public health experts say they are worried the virus could mutate and become more transmissible.

Moderna surged as investors turned eyed potential demand for a vaccine to protect against bird flu after the US reported its first death from the virus.