The H5N1 avian influenza virus is rapidly spreading through dairy cows in the United States, affecting 11 states in four months. This outbreak poses significant human health risks, as the virus can potentially cross species barriers and cause a human epidemic or pandemic. The CDC monitors the situation, considering the overall risk to human health low but noting higher risk for those exposed to infected birds or mammals. Experts stress the need for increased testing and surveillance to prevent a human-adapted strain of bird flu.