Influenza virus demonstrates several levels of antigenic variations, drifts and shifts, which help it to escape to herd immunity pressures. Recent examples show the importance of such variations in the succession and continuity of epidemiological manifestations. During the last few years, simultaneous circulation of several subtypes of influenza A was observed while in the past, one subtype was replacing the other. Thus in 1981, for the only subtype H3N2, A/Texas, A/Bangkok and A/intermediate Texas-Bangkok viruses were isolated in France at the same time in several foci. Furthermore, double infection with different subtypes in the same individual have been documented in man as well as in animals (birds). Related strains have also been isolated simultaneously in the same individual and cloning by plaque analysis makes it possible to evaluate easily this possibility in wild strains. The unexpected frequency of double infections or of variations of virus in feral animals or humans increases the possibility of genetic reassortments and participates for an important part to global variability of the virus.