https://www.theguardian.com/world/2012/mar/28/history-major-flu-pandemics .
Annual flu epidemics kill 250,000-500,000 people each year and cause severe illness in 3 million to 5 million. But new strains that jump from animals to humans can be even more devastating if the global population has no immunity to the virus.
• 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 worldwide. 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.
•1957-58 "Asian flu"
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First identified in China, this H2N2 virus caused roughly 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 global population has no immunity to the strain. Most of those who died were elderly.
• 1968-69 "Hong Kong flu"
The pandemic was first detected in Hong Kong. The H3N2 virus killed around 1 million people globally, with those over 65 most vulnerable. 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 first outbreak in Hong Kong killed six of 18 people infected. The World Health Organisation has recorded 598 cases since 2003, with 352 deaths. Most deaths from bird flu are in Egypt, Indonesia and Vietnam and China. So far, the virus has not adapted to spread easily between humans.
• 2009 "Swine flu"
One of the new strains was the H1N1 "swine flu" virus that originated in Mexico in 2009. The virus, a combination of a Eurasian swine flu virus with another strain that was itself a mix of bird, swine and human flu virus. The strain went on to kill more than 18,000 people around the world.
