Avian flu typically spreads between wild and domestic birds but can spread to other animals and people. There have now been more than 40 confirmed human cases in the United States, according to the Centre for Disease Control,

A teenager in British Columbia was hospitalized in critical condition with the disease, and officials were working to find out how the person was exposed to the virus.

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.