The H5N1 virus that infected a Canadian teen appears not to have spread, but it shows signs of mutation that makes flu virologists nervous.

H5 bird flu, a type of avian influenza, has infected dozens of people in the U.S., and spread to seven states and Canada this year.As concern grows over whether it could lead to another pandemic or lockdown,

In the 2009 swine flu pandemic, multiple reassortment events in pigs and birds led to the novel H1N1 virus strain, which led to 60 million cases and 12,000 deaths in the U.S. in its first year of circulating, per the CDC.