An influenza pandemic is a rare but recurrent event.
Three pandemics occurred in the previous century: “Spanish influenza” in 1918, “Asian influenza” in 1957, and “Hong Kong influenza” in 1968.
The 1918 pandemic killed an estimated 40–50 million people worldwide. That pandemic, which was exceptional, is considered one of the deadliest disease events in human history.
Subsequent pandemics were much milder, with an estimated 2 million deaths in 1957 and one million deaths in 1968.
Modeling research using today's global population has projected that at a minimum, between 2 and 7.4 million people might die in the next pandemic.
More deaths are certainly possible, but until the pandemic strain emerges and we are able to determine its lethality and attack rate, it will be difficult to predict its impact worldwide.
Higher projections of deaths are in general based on extrapolations from the 1918 Spanish flu pandemic. More people died in 1918 from influenza than in a similar period from any other infectious disease ever, including smallpox and the plague.
Thus, because the 1918 pandemic was the single most devastating infectious disease outbreak ever recorded, WHO does not feel it is appropriate to project future pandemic numbers based on such an exceptional event.