Most human cases of bird flu in North America have been mild, a fact that’s underscored by a new study of the first 46 confirmed human H5N1 infections in the United States this year. But the case of an ill Canadian teen stands out because of its severity and because the source of exposure remains a mystery.

The CDC has confirmed a total of 66 human cases of bird flu across the United States as of Friday, although the number is believed to be higher.

In a positive finding, the CDC emphasized that there is no evidence the virus has spread from the Louisiana patient to others, and the mutations alone are unlikely to spread from person to person. CDC scientists also confirmed, however, that the Louisiana virus samples are closely related to candidate vaccine strains already developed for bird flu.

The patient in Louisiana who was hospitalized with severe bird flu illness was found to have a mutated version of the virus. Doctors react and share their concerns.

As of December 30, the CDC has confirmed 66 human cases of the bird flu virus in the United States. Most of them were mild infections among dairy workers and others who had contact with infected animals.

In 2024, at least 66 human cases of bird flu have been reported in the United States, each one raising the risk of another pandemic like COVID-19. So far, most cases have been described as “mild,” which means they weren't hospitalized.