Spanish flu, also known as the 1918 pandemic, is a deadly pandemic caused by the H1N1 influenza virus. From February 1918 to April 1920, 500 million people were infected four times in a row, which corresponds to a third of the world population at that time. Deaths are usually estimated at 20 to 50 million, but estimates range from 17 to 100 million, making this epidemic one of the deadliest in human history.