Influenza virus as an enveloped virus is relatively vulnerable to damaging environmental impacts. Depending on environmental conditions (e.g. humidity and temperature), however, it can survive up to several hours and in water at low temperatures (e.g. <20 °C) also considerably longer (up to several months). Influenza viruses are sensitive to lipid solvents and detergents. They are also vulnerable to heat and a low pH, depending on the virus type. Influenza A viruses with uncleaved HA are obviously more stable (loss of infectivity at pH <4.5) than viruses with cleaved HA (loss of infectivity at pH <5) The most outstanding characteristic of influenza viruses is their rapid evolution which leads to its great variability. This is the case especially with influenza A viruses. According to the antigenic properties of their envelope proteins, influenza A viruses are subdivided into a number of subtypes. 16 different HA and 9 different NA subtypes have been identified so far. The nomenclature system follows the pattern H(x)N(y) including the host of origin, geographical location, strain number, and year of isolation Influenza B viruses are not further divided into subtypes. The accumulation of point mutations leads to a step-by-step modification of the virus proteins (above all in the two surface antigens HA and NA). This mechanism is described as ‘antigen drift’ and is also typical of influenza B viruses