How Influenza Pandemics Occur

This video from the National Institute of Allergy and Infectious Diseases discusses the causes of influenza pandemics.

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.