MIT researchers have explained why two mutations in the H1N1 avian flu virus were critical for viral transmission in humans during the 1918 pandemic outbreak that killed at least 50 million people -believed more than that taken by the Black Death, and higher than the number killed in World War I.

The 1918 flu pandemic -commonly known as the Spanish flu- was an influenza pandemic that started in the United States, appeared in West Africa and France and then spread to nearly every part of the globe in three waves lasting from March 1918 to June 1920, spreading to the Arctic and remote Pacific islands. It was caused by an unusually severe and deadly Influenza A virus strain of subtype H1N1. In contrast to most influenza outbreaks which predominantly affect juvenile, elderly, or otherwise weakened patients, the Spanish Flu also claimed healthy young adults, resulting from infection rates of up to 50% and the extreme severity of the symptoms, suspected to be caused by cytokine storms.

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