Classical avian influenza ("bird flu") is a particularly severe form of avian influenza in poultry and other birds caused by highly pathogenic influenza viruses of subtypes H5 and H7. Infections with other subtypes usually remain without serious clinical consequences. Waterfowl form a natural virus reservoir, especially for low pathogenic influenza viruses. Such low pathogenic influenza viruses can transform to the highly pathogenic form in farm poultry such as chickens and turkeys; then the clinical picture of avian influenza appears.