Avian influenza is a contagious disease with zoonotic potential caused by a virus from the genus Influenzavirus A, family Orthomyxoviridae. These viruses, which are divided into lowpathogenic and highly-pathogenic strains, are characterized by high variability and a propensity for mutations that can clinically differently manifest. Highly pathogenic strains of avian influenza are developed by mutations of low pathogenic strains of subtypes H5 and H7. The most common natural hosts are ducks, and aberrant hosts are chickens, turkeys and mammals such as pigs, horses and humans. The natural hosts represent reservoirs of this disease and are a source of infection for other birds, domestic mammals and humans. In most cases, the natural hosts do not develop severe clinical signs, moreover, they are mostly asymptomatic carriers of the infection. The clinical signs depend on the strain of the virus, environmental factors, secondary infections and the type, age, sex and immune status of the host. Both types of strains are continuously monitored within the populations of poultry, domestic birds, game and wild birds. The occurrence of influenza results in killing and safe disposal of the entire poultry population of an area, therefore this disease causes great economic losses and takes on an increasing economic and public health impact. The disease is an omnipresent danger for poultry production, as evidenced by epidemics in the past decades, continuous presence of highly pathogenic viruses in wild birds as well as new by recently confirmed cases of influenza in poultry in Croatia and Europe. In the Republic of Croatia, the occurrence of influenza in wild birds has been monitored since 2005, when the first cases of this disease were recorded. Among wild species in Croatia, the virus is most often detected in dead swans, then in other sensitive species such as wild geese, grey herons and great cormorants. In recent years, highly pathogenic viruses of H5N1 and H5N8, have most often appeared in wild birds and domestic poultry in Croatia.