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.

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.

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