In the 20th century, the appearance of new subtypes of the influenza virus caused three pandemics, which have spread throughout the world in less than 1 year.
The famous 1918 Spanish flu that killed 50 million people isn't the only pandemic that caused a global health crisis in the past century. Two other pandemics, though much milder than the more well-known one, also strained national health-care systems, researchers say. The 1957 Asian flu killed at least 2 million, and about 700,000 died during the 1968 Hong Kong flu.
Although the origin of the 1918-19 pandemic is not clear, recent pathology analysis of 1918/19 tissue samples from US WWI soldiers place the likely beginnings in Kansas, where the virus was likely passed from birds to swine to people. It then travelled easily across the Atlantic with troop deployment and press censorship during the war prevented information from being shared among countries and scientists, except in Spain, which was a neutral country and open to sharing information on their pandemic – hence the reference to “Spanish”. We now know that the 1957-58 and the 1968-69 pandemics were both caused by viruses which contained a genetic combination of information from a human influenza virus and an avian one.