This video from the National Institute of Allergy and Infectious Diseases discusses the causes of influenza pandemics. When 2 different influenza A viruses infect a single animal, the narrator explains, they can combine to form a new virus to which people have limited immunity. There were 3 influenza pandemics during the 20th century, in 1918, 1957, and 1968. The first pandemic of the 21st century, the H1N1 pandemic of 2009, is thought to have occurred due to 2 viruses circulating in pigs that created a new virus capable of infecting people.
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