Influenza pandemic
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An influenza pandemic is an epidemic of an influenza virus that spreads across a large region and infects a large proportion of the population. There have been six major influenza epidemics in the last 140 years, with the 1918 flu pandemic being the most severe; this is estimated to have been responsible for the deaths of 50–100 million people. The 2009 swine flu pandemic resulted in under 300,000 deaths and is considered relatively mild. These pandemics occur irregularly.
Influenza ward at Walter Reed Hospital, in Washington, D.C., during the 1918 flu pandemic.
Estimates of hypothetical influenza deaths in the 2010 United States population (308,745,538 persons) across varying values of case-fatality ratio and the cumulative incidence of infection in the population. Selected estimated numbers of deaths are indicated with a black line, across each relevant combination of case-fatality ratio and cumulative incidence. In addition, the background color transitions from blue to yellow to red as the estimated absolute number of deaths increases. Case-fatality ratio is an example of a clinical severity measure and cumulative incidence of infection is an example of a transmissibility measure in the Pandemic Severity Assessment Framework.
Scaled examples of past influenza pandemics and past influenza seasons. Color scheme included to represent corresponding hypothetical estimates of influenza deaths in the 2010 US population, with the same color scale as the previous figure.
Influenza intervals in the CDC's Pandemic Intervals Framework
Spanish flu
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The 1918–1920 flu pandemic, also known as the Great Influenza epidemic or by the common misnomer Spanish flu, was an exceptionally deadly global influenza pandemic caused by the H1N1 influenza A virus. The earliest documented case was March 1918 in the state of Kansas in the United States, with further cases recorded in France, Germany and the United Kingdom in April. Two years later, nearly a third of the global population, or an estimated 500 million people, had been infected in four successive waves. Estimates of deaths range from 17 million to 50 million, and possibly as high as 100 million, making it one of the deadliest pandemics in history.
Soldiers sick with Spanish flu at a hospital ward at Camp Funston in Fort Riley, Kansas
El Sol (Madrid), 28 May 1918: "The three-day fever – In Madrid 80,000 Are Infected – H.M. the King is sick"
Front page of The Times (London), 25 June 1918: "The Spanish Influenza"
Advertisement in The Times, 28 June 1918 for Formamint tablets to prevent "Spanish influenza"