Global pandemics have been observed for several hundred years. The best documented pandemics occurred in 1918 (A(H1N1), Spanish flu), 1957 (A(H2N2), Asian flu) and 1968 (A(H3N2), Hong Kong flu). All varied in severity.

Spanish influenza is estimated to have infected 50% of the world's population of which half suffered a clinical infection and the total excess mortality was estimated of 40-50 million. This gives a mortality rate of 2-2.5%. The attack and mortality rates were highest among healthy adults (20-40 years old).

Asian influenza affected some 40-50% of people during two waves of the pandemic, of which 25-30% experienced clinical disease. The mortality rate was estimated at approximately 1 in 4000 and the total death toll probably exceeded 1 million.

The morbidity and mortality of Hong Kong influenza was as high as in the 1957-1958 pandemic and showed a range of 1-3 million fatalities. Both Asian and Hong Kong influenza arose in South-East Asia.