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

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.