Avian influenza: A highly contagious viral disease with up to 100 percent mortality in domestic fowl. Caused by influenza A virus subtypes H5 and H7. All types of birds are susceptible to the virus, but outbreaks occur most often in chickens and turkeys. The infection may be brought by migratory wild birds which can carry the virus, but show no signs of disease. Humans are only rarely affected. Also known as fowl plague, avian flu, and bird flu.