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.