The 1918 flu pandemic (the Spanish Flu) was an unusually severe and deadly influenza pandemic that spread across the world. Historical and epidemiological data are inadequate to identify the geographic origin. Most victims were healthy young adults, in contrast to most influenza outbreaks which predominantly affect juvenile, elderly, or weakened patients. The flu pandemic was implicated in the outbreak of encephalitis lethargica in the 1920s.

The pandemic lasted from June 1917 to December 1920, spreading even to the Arctic and remote Pacific islands. Between 50 and 100 million died, making it one of the deadliest natural disasters in human history. An estimated 50 million people, about 3% of the world's population (1.8 billion at the time), died of the disease. Some 500 million, or 28% (≈1/4) were infected.

Figure 1. Soldiers from Fort Riley, Kansas ill with Spanish influenza at a hospital ward at Camp Funston in 1918, and two American Red Cross nurses demonstrate treatment practices during the influenza pandemic of 1918.

 

Tissue samples from frozen victims were used to reproduce the virus for study. Among the conclusions of this research is that the virus kills via a cytokine storm (overreaction of the body's immune system) which perhaps explains its unusually severe nature and the concentrated age profile of its victims. The strong immune systems of young adults ravaged the body, whereas the weaker immune systems of children and middle-aged adults resulted in fewer deaths.

 

Although the first cases were registered in the continental U.S. and the rest of Europe long before getting to Spain, the 1918 pandemic received its nickname "Spanish flu" because Spain, a neutral country in WWI, had no censorship of news regarding the disease and its consequences. Spanish King Alfonso XIII became gravely ill and was the highest-profile patient about whom there was coverage, hence the widest and most reliable news coverage came from Spain, giving the false impression that Spain was most affected.

Video 1: The Influenza Pandemic of 1918 (YouTube, 2:37)