Federal health officials say that not only has a Louisiana resident contracted the first severe case of bird flu in the U.S., but that the virus likely mutated in the patient. The findings raise concern regarding the adaptability of the virus.

H5N1 usually spreads to people through contact with infected or dead birds, as with the Louisiana patient. Currently, there are no known cases of human-to-human transmission, said the CDC's Dr. Demetre Daskalakis.