Accurate information on the epidemiology of influenza over the World is difficult to obtain as it is dependent on both national statistics of morbidity and mortality and on adequate virological confirmation of suspected cases. Only a few countries have the facilities for these and in much of the World it is difficult to establish exactly the impact of influenza. For this reason, our knowledge on the natural history of the disease and the virus causing it is restricted to what happens in a few areas of the World where the necessary information is obtainable. In one of these areas, the U.K., it has been possible to follow the appearance of influenza in successive years throughout the 11 years of prevalence of the H2N2 virus and throughout the 10 years of prevalence of the H3N2 virus. What are still unknown are the factors that determine this pattern. Various hypotheses have been proposed to explain the epidemiology of influenza.