The bird flu virus that brought a Canadian teen to the “brink of death” may have mutated while inside her body to become more efficient at spreading among humans. The case, and the death of a Louisiana resident over the age of 65 who was hospitalized with a severe H5N1 avian flu infection in December, are stark reminders “that this is not a mild virus,” said Matthew Miller, Canada Research Chair in viral pandemics at McMaster University in Hamilton. The Louisiana fatality marks the first U.S. death from highly pathogenic avian influenza. The B.C. girl’s infection is Canada’s first domestically acquired case of H5N1.