The epidemiology of inter-pandemic influenza (also named seasonal influenza) is characterized in temperate zones by epidemics of variable size that occur during the colder winter months (November to April in the Northern Hemisphere and May to September in the Southern Hemisphere), each of which typically lasts 8–10 weeks . In a study on influenza activity throughout eight seasons (1999–2007), the average length of epidemics in 23 European countries was 15.6 weeks (median 15 weeks; range 12–19 weeks) .