Q&A: Bird flu basics from an infectious disease expert

The H5N1 Avian influenza is decimating bird populations around the world. More than 136 million birds have been affected in the US since the disease arrived in 2022. More than 5 million egg-laying chickens died in the first 16 days of 2025. It also shows signs of mutating to become more infectious to mammals. The disease has shown up in dozens of cats, zoo animals, people and more than 900 dairy herds.