Bird flu is an infection caused by a type of avian influenza virus. Although there are many types of bird flu, the types that now concern health workers—because they are very serious and often fatal—are the H5N1 and H7N9 bird flu viruses.

Symptoms of bird flu can be the same as common flu symptoms. These can include a fever, cough, sore throat, muscle aches, or eye infection (conjunctivitis). More serious symptoms of bird flu include pneumonia, a serious lung infection, or acute respiratory distress syndrome, a life-threatening lung problem.