H5N1 avian influenza, more commonly known as bird flu, has infected more than 100 million birds in the U.S. and almost 500 dairy cattle herds across 15 states. The virus has popped up in mammals including elephant seals, goats, foxes, and house cats.

Health officials were struggling to answer a broader and potentially worrisome question: how the infection occurred.

A teenager is in critical condition in a British Columbia children’s hospital with what is likely Canada’s first human case of H5 bird flu

A Canadian teenager is hospitalized in critical condition with bird flu, health officials reported Tuesday. The teen has been receiving care at BC Children’s Hospital in Vancouver since Friday, the same day an initial test came back positive for H5 influenza.

A Canadian teen is hospitalized in critical condition with what is believed to be bird flu, a British Columbia health official said Tuesday. It’s not clear how the teenager picked up the virus, which has been detected recently in wild birds and poultry in the province,

A Canadian teen is severely ill with what is believed to be bird flu. The teen is not known to have had any contact with infected animals.