Cases of H5N1 bird flu in U.S. dairy and poultry workers have largely been mild. However, a new case in a British Columbia teenager has experts worried.

As a Canadian teenager’s condition worsened amid the first presumptive positive test for bird flu in that country in over a decade, 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.