Influenza epidemic of 1918

The 1918 flu pandemic (January 1918 – December 1920) was an unusually deadly influenza pandemic, the first of the two pandemics involving H1N1 influenza virus. It infected 500 million people across the world, including remote Pacific islands and 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. A considerable spike occurred at the time of the pandemic, specifically the year 1918. Life expectancy in the United States alone 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 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 original 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.

Historical and epidemiological data are inadequate to identify the pandemic's geographic origin. It was implicated in the outbreak of encephalitis lethargica in the 1920s.

To maintain morale, wartime censors minimized early reports of illness and mortality in Germany, Britain, France, and the United States; but papers were free to report the epidemic's effects in neutral Spain (such as the grave illness of King Alfonso XIII), creating a false impression of Spain as especially hard hit—thus the pandemic's nickname Spanish flu. In Spain, a different nickname was adopted, the Naples Soldier (Soldado de Nápoles), which came from a musical operetta (zarzuela) titled La canción del olvido (The Song of Forgetting), which premiered in Madrid during the first epidemic wave. Federico Romero, one of the librettists, quipped that the play's most popular musical number, Naples Soldier, was as catchy as the flu.

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