Avian influenza, or bird flu as it is more commonly known, is a noninfectious disease for most species. All known bird flu viruses are from the "Influenza A" species of virus. Migratory waterfowl and ducks act as the culprits by being the carriers of the virus that cause bird flu. Often they are unaffected by the virus and show no signs of disease. These carriers can spread the virus to susceptible species, particularly domestic chickens, turkeys and geese. The virus usually then quickly mutates and can kill over 90% of the total flock. In general the avian virus does not affect the human population, but in Hong Kong, in1997, a bird flu outbreak infected 18 people, killing 6 of them. Thereafter there have been a number of cases seen in Asia, in Europe and in the Middle East. Two particularly dangerous influenza subtypes are (A) H7N7 and (A) H5N1.