Pathogenicity refers to the ability of an infectious agent such as a virus to cause disease. Avian influenza (AI) viruses can be classified as either low pathogenic avian influenza (LPAI) or highly pathogenic avian influenza (HPAI), see AI Classifications for more information. Most AI viruses are considered low pathogenic, meaning they only cause mild clinical signs in a few birds.
In contrast, HPAI infects birds severely and quickly, resulting in rapid death losses due to the disease itself and to the culling of infected flocks. In rare circumstances, the highly pathogenic H5N1 virus, also known as bird flu, has infected and killed humans, with the first reported case in Hong Kong in 1997, see confirmed human cases of avian influenza and the history of avian influenza for additional information.