Genetic sequences of H5N1 bird flu viruses collected from a person in Louisiana who became severely ill show signs of development of several mutations thought to affect the virus’ ability to attach to cells in the upper airways of humans, the Centers for Disease Control and Prevention reported on Thursday.

One of the mutations was also seen in a virus sample taken from a teenager in British Columbia who was in critical condition in a Vancouver hospital for weeks after contracting H5N1.

The mutation seen in both viruses is believed to help H5N1 adapt to be able to bind to cell receptors found in the upper respiratory tracts of people. Bird flu viruses normally attach to a type of cell receptor that is rare in human upper airways, which is believed to be one of the reasons why H5N1 doesn’t easily infect people and does not spread from person-to-person when it does.