The 1918 flu pandemic (January 1918 – December 1920), also known as the Spanish flu, was an unusually deadly influenza pandemic, the first of the two pandemics involving H1N1 influenza virus. It infected 500 million people around the world, including people on remote Pacific islands and in the Arctic, and resulted in the deaths of 50 to 100 million (three to five percent of the world's population), making it one of the deadliest natural disasters in human history. Disease had already greatly limited life expectancy in the early 20th century. In the first year of the pandemic, life expectancy in the United States dropped by about 12 years. Most influenza outbreaks disproportionately kill juvenile, elderly, or already weakened patients; in contrast, the 1918 pandemic predominantly killed previously healthy young adults. There are several possible explanations for the high mortality rate of the 1918 influenza pandemic. Some research suggests that the specific variant of the virus had an unusually aggressive nature. One group of researchers recovered the virus from the bodies of frozen victims, and found that transfection in animals caused a rapid progressive respiratory failure and death through a cytokine storm (overreaction of the body's immune system). It was then postulated that the strong immune reactions of young adults ravaged the body, whereas the weaker immune systems of children and middle-aged adults resulted in fewer deaths among those groups. More recent investigations, mainly based on original medical reports from the period of the pandemic, found that the viral infection itself was not more aggressive than any previous influenza, but that the special circumstances (malnourishment, overcrowded medical camps and hospitals, poor hygiene) promoted bacterial superinfection that killed most of the victims typically after a somewhat prolonged death bed.
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