Influenza A viruses that infect poultry can be divided into two groups. Very virulent viruses cause highly pathogenic avian influenza (HPAI), with flock mortality as high as 100%. These viruses have been restricted to subtypes H5 and H7, although not all H5 and H7 viruses cause HPAI. All other viruses cause a milder, primarily respiratory, disease (LPAI), unless exacerbated. Until recently, HPAI viruses were rarely isolated from wild birds, but for LPAI viruses extremely high isolation rates have been recorded in surveillance studies. Influenza viruses may infect all types of domestic or captive birds in all areas of the world. The frequency with which primary infections occur in any type of bird usually depends on the degree of contact there is with feral birds. Secondary spread is typically associated with human involvement, either by birds or bird product movement or by transferring infective faeces from infected to susceptible birds, but potentially wild birds can be involved. In recent years the frequency of HPAI outbreaks appears to have increased and there have been particularly costly outbreaks of HPAI in densely populated poultry areas in Italy, The Netherlands and Canada. In each outbreak millions of birds were slaughtered to bring the outbreaks under control. Since the 1990s, AI infections due to two subtypes have been widespread in poultry across a large area of the World. LPAI H9N2 appears to have spread across the whole of Asia in that time and has become endemic in poultry in many of the affected countries. However, these outbreaks have been overshadowed by the H5N1 HPAI virus, initially isolated in China that has now spread in poultry and/or wild birds throughout Asia and into Europe and Africa, resulting in the death or culling of hundreds of millions of poultry and posing a significant zoonosis threat. To date control methods seem to have been unsuccessful on the larger scale and HPAI H5N1 outbreaks continue to be reported.