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,

Canadian health officials have confirmed bird flu in a British Columbia teen. The Public Health Agency of Canada said Wednesday that tests showed the virus involved was related to a bird flu outbreak in poultry in the province.

A Canadian teenager infected with bird flu — that country’s first case involving a locally acquired infection — is in critical condition and experiencing difficulty breathing, health officials said Tuesday.