Influenza virus genome consists of 7 independent DNA molecules. It is necessary exactly 7 fragments for the further reproductive ability This genetic material has information mostly about the surface proteins: the hemagglutinins (H) and the neuraminidases (N). A reassortment is a phenomena when different types of influenza virus (from the different species) exchange some fragments of DNA among themselves during a cell infection by various influenza viruses. In the total, the offsprings of these viruses will have 7 segments genome each, but in the new DNA combinations. It is a way of the new influenza virus modifications, which are potentially very dangerous. In such way nature has presented us H1N1, which genome includes fragments from birds, pigs and human influenza viruses.