Avian influenza, first identified in Italy more than 100 years ago, affects birds. Migratory wildfowl, particularly wild ducks, are the natural reservoir. As with many animal diseases, humans can sporadically become infected under special conditions. Most recently, avian influenza A (H7N9) has been reported in China and also has been detected in poultry in China. While mild illness in human cases has been seen, most patients have had severe respiratory illness and some people have died. No cases of H7N9 outside of China have been reported. The new H7N9 virus has not been detected in people or birds in the United States.
Since 2003, H5N1, a highly pathogenic avian (bird) flu virus has caused serious outbreaks in domestic poultry in parts of Asia and the Middle East. Highly pathogenic refers to the virus's ability to produce diseases. Although H5N1 does not usually infect humans, nearly 600 human cases of H5N1 have been reported over the past decade in 15 countries, but there have been no reported infections in birds, poultry or people in the United States. Most human cases of H5N1 virus infection have occurred in people who had recent contact with sick or dead poultry that were infected with H5N1 viruses. About 60 percent of people infected with the virus died from their illness. Unlike other types of flu, H5N1 usually does not spread between people.
Avian Influenza - USDA
Traveler's Health - CDC