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The Spanish flu (1918-1919) was exceptionally severe, and death rates were higher than with any other influenza pandemic to date. An estimated 500 million people were infected during this pandemic. This flu is not likely to re-emerge. Instead, experts believe a pandemic will likely be caused by an influenza subtype to which there is little, or no, preexisting immunity in humans.
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 Spanish flu, and others died of related complications. Nearly half of those who died were young, healthy adults.
The Impact of the Spanish Flu
An estimated one-third of the world's population (500 million people) were infected and had flu symptoms during the Spanish flu. The Spanish flu was exceptionally severe, and death rates were higher than with any other influenza pandemic. It is estimated that the total number of deaths from the Spanish flu ranged from 50 million to 100 million people.
The specific virus that caused the Spanish 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 Spanish 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 human disease. Examination of mortality data from the 1920s suggested that within a few years after the Spanish flu, influenza epidemics had settled down, with substantially lowered death rates.