Another cause for concern: flu season is underway in the U.S., and as seasonal influenza virus goes around, “humans, ourselves, could be a mixing vessel,” Davis says. If a person were simultaneously infected with bird flu and seasonal flu, the two viruses could theoretically combine to create a more transmissible strain.

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

British Columbia health officials have yet to identify a likely source of the infection, though none of the teen's contacts have tested positive for the virus so far.

A Canadian teen is hospitalized in critical condition with what is believed to be bird flu. British Columbia health officials said Tuesday it’s not clear how the teen picked up the virus .

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