The death in Louisiana — involving a person with underlying health conditions — is the first human case in the United States linked to exposure to backyard birds, according to the federal Centers for Disease Control and Prevention.

Bird flu continues to spread across the United States after jumping from chickens and turkeys to cow milk and humans. The first death was reported this week in Louisiana after a 65-year-old man with underlying medical conditions succumbed to the flu.

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.