A teenager in Canada is critically ill with the first human case of H5N1 bird flu contracted in the country, and public health officials aren't sure how the youth was exposed.
The teen had no known exposure to livestock such as cattle or poultry, where the virus is known to be circulating. The strain that sickened the teen, however, is different from the version that is infecting cows and has spread to dozens of dairy workers in the U.S.
