Flu epidemics kill 250,000 to 500,000 people each year and cause severe illness in 3 million to 5 million. However, new strains that jump from animals to humans can be even more devastating if the population has no immunity.
1918 “Spanish flu” — The most devastating flu pandemic in recent history was caused by a strain of H1N1 influenza that killed 20 million to 50 million people. The pandemic spread in three more or less simultaneous waves through Europe, Asia and North America. Unusually, the virus was particularly lethal in young adults. The origins of the virus are still unknown.
1957-1958 “Asian flu” — First identified in China, this H2N2 virus caused about 2 million deaths worldwide. The virus is thought to have emerged after a human form of H2N2 combined with a mutant strain in ducks. The strain has not circulated in humans since 1968, so much of the population has no immunity.
1968-1969 “Hong Kong flu” — The pandemic was first detected in Hong Kong. The H3N2 virus killed about 1 million people globally. H3N2 viruses still circulate today.
1997 “Bird flu” — For the first time, an influenza virus was found to spread directly from birds to people. The H5N1 bird flu infections were linked to poultry markets. The WHO has recorded 598 cases since 2003, with 352 deaths.
2009 “Swine flu” — The H1N1 “swine flu” virus originated in Mexico in 2009, a combination of a Eurasian swine flu virus with another strain that was a mix of bird, swine and human flu. It went on to kill more than 18,000 people.