Spanish flu, also known as the Great Influenza epidemic or the 1918 influenza pandemic, was an exceptionally deadly global influenza pandemic caused by the H1N1 influenza A virus. The earliest documented case was March 1918 in Kansas, 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 17.4 million to 100 million, with an accepted general range of 25–50 million, making it one of the deadliest pandemics in human history.

The name "Spanish flu" is a misnomer, rooted in historical othering of infectious disease origin, which is now avoided. The pandemic broke out near the end of World War I, when wartime censors suppressed bad news in the belligerent countries to maintain morale, but newspapers freely reported the outbreak in neutral Spain. These stories created a false impression of Spain as the epicenter, so press outside Spain adopted the name "Spanish" flu. Limited historical epidemiological data make the pandemic's geographic origin indeterminate, with competing hypotheses on the initial spread.

Most influenza outbreaks disproportionately kill the young and old, with a higher survival rate in-between, but this pandemic had unusually high mortality for young adults. Scientists offer several explanations for the high mortality, including a six-year climate anomaly affecting migration of disease vectors with increased likelihood of spread through bodies of water. The virus was particularly deadly because it triggered a cytokine storm, ravaging the stronger immune system of young adults, although the viral infection was apparently no more aggressive than previous influenza strains. Malnourishment, overcrowded medical camps and hospitals, and poor hygiene, exacerbated by the war, promoted bacterial superinfection, killing most of the victims after a typically prolonged death bed.

The 1918 Spanish flu was the first of three flu pandemics caused by H1N1 influenza A virus; the most recent one was the 2009 swine flu pandemic. The 1977 Russian flu was also caused by H1N1 virus, but it mostly affected younger populations. The ongoing pandemic of COVID-19, which began in December 2019 and is caused by Severe acute respiratory syndrome coronavirus 2, is the deadliest respiratory virus pandemic since Spanish flu due to its spread worldwide and high death toll.

 

Note:   The above text is excerpted from the Wikipedia article Spanish flu, which has been released under the GNU Free Documentation License.