Influenza A type viruses comprise a diverse group of viruses that affect particularly aquatic birds. The majority of their subtypes are low pathogenic, but two subtypes (H5 and H7) include also viruses that can cause severe epidemics of avian influenza and major financial losses to the poultry industry. Some avian influenza viruses, such as H5N1, have the ability to infect humans. Infections are very rare. Almost all cases of human infections have been caused through handling of poultry infected or killed by avian influenza or the excretions of affected birds in poor hygienic conditions.