The five people were likely infected by chickens, which they had been tasked with killing in response to a bird flu outbreak at the farm.

More than 99 million chickens and turkeys have been infected with a highly pathogenic strain of the bird flu that emerged at U.S. poultry farms in early 2022.

The federal government has compensated poultry farmers more than $1 billion for destroying infected flocks and eggs to keep outbreaks from spreading.

The H5N1 bird flu virus has spread among poultry farms around the world for nearly 30 years. An estimated 900 people have been infected by birds, and roughly half have died from the disease.

The virus made an unprecedented shift this year to dairy cattle in the U.S. This poses a higher threat because it means the virus has adapted to replicate within cows' cells, which are more like human cells.

Scientists have warned that the virus could mutate to spread from person to person, like the seasonal flu, and spark a pandemic.

The CDC has controversially decided not to offer at-risk groups bird flu vaccines. Demetre Daskalakis, director of the CDC's National Center for Immunization and Respiratory Diseases, told KFF Health News that the agency is not recommending a vaccine campaign at this point for several reasons, even though millions of doses are available.

To understand whether cases are going undetected, researchers in Michigan have sent the CDC blood samples from workers on dairy farms. If they detect bird flu antibodies, it's likely that people are more easily infected by cattle than previously believed.