Epidemics that were probably influenza have been reported throughout recorded history. There were 13 fairly severe epidemics during the 18th century and 12 during the 19th century. Probably 8 of these 25 were influenza pandemics. In the 20th century there have been 4 pandemics (1918/19, 1957/58, 1968/69 and 1977) due to the emergence of new subtypes of influenza A virus. The great pandemic of 1918/19 caused an estimated 20 million deaths. Between pandemics usually there have been epidemics of varying severity (...) at intervals of one to three years and a trickle of sporadic cases every winter. The morbidity and mortality rates have varied greatly from epidemic to epidemic and from place to place during the same epidemic. Generally the morbidity has been lowest in people over 60 years of age, but, except for 1918/19, the mortality has been predominantly in old people. The epidemic behaviour of influenza has been so erratic and difficult to understand that there are still a few scientists who consider that extraterrestrial influences operate. These views are not taken seriously by most virologists but there are puzzling aspects of influenza that are not yet understood. (shrink)