Influenza A virus (Influenzavirus A) belongs to the Orthomyxoviridae family of viruses which is marked by single-stranded segmented RNA. This virus is mainly found in birds, but infection can also occur in humans and mammals (pigs, seals or horses). It is the most frequent cause of a flu epidemic and pandemic in the world. It is marked by high genetic variability which is influenced by such factors as the possibility of exchanging RNA segments among various strains of the viruses (known as antigenic shift). The influenza A virus is divided into subtypes depending on the type of proteins building its protein shell, haemagglutinin (HA) and neuraminidase (NA). So far, 18 subtypes of HA and 11 subtypes of NA have been distinguished. Most well-known is subtype H5N1 that initiated bird flu. Flu viruses are airborne pathogens and attack the ciliated epithelial cells in the respiratory system, which opens up the way to bacterial pathogens (frequent flu complications include bacterial superinfections). The disease is marked by a sudden onset. The first symptoms of the disease include feeling unwell, shivering, headache, fever and bone and joint pain. Frequent are runny nose, sore throat, dry persistent cough and chest pain. The incubation period is very short; after manifestation, the disease develops in 18–72 hours.