The bird flu is an infectious disease of birds caused by an influenza type A virus. For the present, all epidemics of the disease described among birds were caused by highly pathogenic forms of H5 and H7 subtypes. Current pandemic of bird flu is caused by a highly pathogenic H5N1 subtype, which already migrates among wild birds and is transferred quickly by their migration into very remote areas. Water birds form an infection reservoir from which the infection may be spread on poultry, mammals and very rarely on man. All influenza viruses are highly specific for species and therefore the infection transmission on other animal species is relatively rare and mostly under abnormal conditions. Unambiguously, the most suitable conditions for interspecies transmission of an influenza A virus are in some areas of South Eastern Asia. The first cases of an H5N1 virus transmission from chickens to the man were described in Hongkong in 1997. An inter-human transmission of the infection has not been unambiguously proved yet.

        Key words: bird flu, influenza A virus, H5N1 subtype, pandemic