Unravelling the ecology of influenza A virus.

For 20 years after the influenza A virus was discovered in the early 1930s, it was believed to be almost exclusively a human virus. But in the 1950s closely related viruses were discovered in diseases of horses, pigs and birds. Subsequently influenza A viruses were found to occur frequently in many species of birds, particularly ducks, usually without causing disease.

The chronicle of influenza epidemics.

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 pandemic.