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.
Since 2020, a strain of highly pathogenic avian influenza called H5N1 has been decimating both wild and domestic birds. Now it’s spreading among US dairy cows, and has infected US farmworkers exposed to sick cattle and poultry. A man in Mexico