The variability of the type B viruses, however, is also characterised by other mechanisms such as insertion and deletion, as the influenza B lines show which have been co-circulating and stable for more than 20 years The process of ‘antigen shift’ (re-assortment) is defined as the exchange of whole genome segments, above all HA genes, which might result in influenza viruses which have a selective advantage compared with their parent viruses. A prerequisite for this re-assortment is simultaneous infection of a cell by two different influenza A viruses. This will result in a variety of different hybrid viruses with different characteristics which they have received from the parent viruses via the individual genome segments. An example is the occurrence of the influenza A/H2N2 subtype in 1957 which superseded the influenza A/H1N1 virus which was dominant until then Special attention was drawn to the public by the deaths caused in humans by the highly pathogenic avian influenza A/H5N1 subtype. These cases have shown that an entirely new influenza A virus can cause deadly infections in humans. Since the first outbreaks of H5N1-related avian influenza in poultry in South-East Asia in 2003, human H5N1 transmissions with high mortality have occurred in 10 countries. It is possible that this virus will adapt better to humans and will then be able to spread among the human population.