During the 1918 flu, one-third of the world's population was infected. The disease was exceptionally severe, and it is estimated that millions of people died worldwide. It is unlikely that a recurrence of this flu will be seen, since some residual immunity to the virus is present in at least a portion of the human population.
The 1918 flu, or "Spanish flu," caused the highest number of known influenza deaths. More than 500,000 people died in the United States, and up to 50 million people may have died worldwide. Many people died within the first few days after infection with the 1918 flu, and others died of complications. Nearly half of those who died were young, healthy adults.
An estimated one-third of the world's population (500 million people) became infected and had flu symptoms during the 1918 flu outbreak. The 1918 flu was exceptionally severe, and death rates were higher than any other influenza pandemic. It is estimated that the total number of deaths from the 1918 flu ranged from 50 million to 100 million people.
The specific virus that caused the 1918 flu was the influenza A (H1N1) virus, which appears to be an avian-like influenza virus derived from an unknown source.
By the early 1990s, 75 years of research had failed to answer a most basic question about the 1918 flu pandemic: "Why was it so fatal?" No virus from 1918 had been isolated, but all of its apparent descendants caused substantially milder forms of disease. Examination of mortality data from the 1920s suggested that within a few years after the 1918 flu, influenza epidemics had settled down, with substantially lowered death rates.