The characteristics of the avian influenza viruses (AIV) have been presented, including their classification, structure and variability mechanisms. Depending on their pathogenicity they were divided into highly pathogenic (HP) and low pathogenic (LP) pathotypes. The definition of avian influenza according to the directive of the European Union was cited, indicating, among others, that HP strains emerge exclusively from H5 and H7 subtypes. The change takes place in poultry and not in wild birds. The outbreak of HPAI H5N1 in South East Asia at the end of 2003, which spread during subsequent years to Asia, Europe and Africa, has put an increased focus on wild, migratory birds that participated each in year seasonal flights. The cited literature indicates that these birds, representing a very large number of species, are carriers of LPAI strains, not being pathogenic for poultry and man. However, if these birds, particularly ducks, survive infection with highly pathogenic avian influenza strains they are able to fly for long distances, while simultaneously being carriers of HPAI H5N1. In such cases they may spread the pathogenic virus intercontinentally. Because of the difficulties in identification of carriership of avian influenza viruses in migrating birds, their role in spreading HPAI H5N1 is not fully clear and needs further investigation. According to the present point of view the probability of causing avian influenza in poultry in connection with migratory birds ranges from low to high with a high degree of uncertainly. The risk of infection of poultry kept in free-range conditions is high with a medium uncertainty of infection and disease.
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