Influenza Pandemics in Recent History
During the 20th century, the emergence of several new influenza A virus subtypes caused three influenza pandemics, all of which spread around the world within a year of being detected.
1918-1919: The "Spanish Flu" Influenza Pandemic
The 1918-1919 "Spanish flu" [A (H1N1)] caused the highest number of known influenza deaths. However, the actual influenza virus subtype was not detected in the 1918 influenza pandemic. More than 500,000 people died in the United States, and up to 50 million people may have died worldwide. Many people died within the first few days after infection, and others died of secondary complications. Nearly half of those who died were young, healthy adults. Influenza A (H1N1) viruses still circulate today after being introduced again into the human population in 1977.
1957-1958: The "Asian Flu" Influenza Pandemic
The 1957-1958 "Asian flu" [A (H2N2)] caused approximately 70,000 deaths in the United States. The Asian flu was first identified in China in late February 1957, and then spread to the United States by June 1957.
1968-1969: The "Hong Kong Flu" Influenza Pandemic
The 1968-1969 "Hong Kong flu" [A (H3N2)] caused approximately 34,000 deaths in the United States. This virus was first detected in Hong Kong in early 1968 and spread to the United States later that year. Influenza A (H3N2) viruses still circulate today.
Viruses containing a combination of genes from a human influenza virus and an avian influenza virus caused both the 1957-1958 and 1968-1969 pandemics. The 1918-1919 pandemic virus appears to have an avian origin as well.