Avian influenza is an infection caused by avian (bird) influenza (flu) viruses. These influenza viruses occur naturally among birds. Wild birds worldwide carry the viruses in their intestines, but usually do not get sick from them. However, avian influenza is very contagious among birds and can make some domesticated birds, including chickens, ducks, and turkeys, very sick and kill them. Influenza A (H5N1) is an influenza A virus subtype that occurs mainly in birds, is highly contagious among birds, and can be deadly to them. Outbreaks of H5N1 among poultry are ongoing in Asia and parts of Europe and Africa. While H5N1 does not usually infect people, human cases of H5N1 infection associated with these outbreaks have been reported in some countries. Most of these cases have occurred from direct or close contact with infected poultry or contaminated surfaces; however, a few rare cases of human-to-human spread of H5N1 virus have occurred, though transmission has not continued beyond one person. Scientists are concerned that the H5N1 virus one day could be able to spread easily from one person to another. Because these viruses do not commonly infect humans, there is little or no immune protection against them in the human population and an influenza pandemic (worldwide outbreak of disease) could begin. Experts from around the world are watching the H5N1 situation very closely and are preparing for the possibility that the virus may begin to spread more easily from person to person, and possibly start a human influenza pandemic, or worldwide outbreak. For information on pandemic influenza, click here.