The 1918 influenza pandemic was the most severe pandemic in recent history. It was caused by an H1N1 virus with genes of avian origin. it spread worldwide during 1918-1919. In the United States, it was first identified in military personnel in spring 1918. While influenza is now often thought of as a common but mild disease, it still kills more than thirty thousand people in the United States each year
It is estimated that about 500 million people or one-third of the world's population became infected with this virus. The number of deaths was estimated to be at least 50 million worldwide with about 675,000 occurring in the United States.
Mortality was high in people younger than 5 years old, 20-40 years old, and 65 years and older. The high mortality in healthy people, including those in the 20-40 year age group, was a unique feature of this pandemic.