1918 Influenza Pandemic

The 1918 influenza pandemic was caused by an influenza A virus of the H1N1 subtype. It spread worldwide during 1918–1919 and is considered one of the deadliest pandemics in recorded history. The name ‘Spanish flu’ does not establish that the virus originated in Spain: wartime censorship limited reporting in belligerent countries, while Spain's relatively open press reported the outbreak, making it especially visible there.

The United States Centers for Disease Control and Prevention reports that influenza was first identified among U.S. military personnel in spring 1918 and that the pandemic spread worldwide in 1918–1919. Historical accounts commonly describe successive waves, with more severe outbreaks in autumn 1918 and winter 1918–19. Wartime troop movements, crowded military camps, ships and expanding transport networks helped move infections between regions. Evidence does not establish a single certain geographic origin; researchers continue to debate where the virus first emerged.

The CDC estimates that about 500 million people—roughly one-third of the world's population at the time—became infected, and that at least 50 million died worldwide, including about 675,000 in the United States. Estimates in historical and scientific sources vary because surveillance and diagnostic systems were limited and many deaths were not recorded consistently. A notable feature was substantial mortality among otherwise healthy young adults, in addition to the high risks generally faced by very young children and older people.

In 1918 there was no influenza vaccine to prevent infection and no antibiotics to treat secondary bacterial infections associated with influenza. Public-health authorities relied on measures such as isolation, quarantine, hygiene, disinfection and restrictions on public gatherings. These interventions were applied unevenly and varied by location. The pandemic's scale exposed the limits of health systems during a fast-moving international epidemic.

The outbreak encouraged later research into influenza viruses, their transmission and pandemic preparedness. A CDC-published scientific review describes how researchers used preserved autopsy material to sequence the 1918 virus and explains that key questions—including the virus's origin and the biological reasons for its unusually high severity—have remained subjects of study. Historical estimates should be treated as approximate rather than exact counts.