Avian influenza viruses in poultry occur in two main forms.
Current research has shown that H5 and H7 viruses are introduced to poultry flocks in their low pathogenic form. If allowed to circulate in the flocks, the viruses can mutate, usually within a few months, into the highly pathogenic form. That is why the presence of an H5 or H7 virus in poultry is always cause for concern even when initial signs of infection are mild.
In rare cases, a highly pathogenic strain of H5N1 which kills poultry has infected and killed some humans. This type of HPAI has been referred to as bird flu.