Bird flu is a disease caused by avian influenza A viruses that usually spread between birds, not people. One subtype of bird flu, caused by avian influenza A(H5) virus, is spreading worldwide in wild birds and causing sporadic outbreaks in U.S. poultry and dairy cows. In addition to spreading to an increasing number of mammals, H5 bird flu has caused some rare human infections. Click here to learn more about bird flu and how to report a sick or dead bird.