In 1918 the world saw a particularly lethal outbreak of the flu, also referred to as the “Spanish flu.” Despite its name, the outbreak seems to have originated from U.S. soldiers returning home in the final days of World War 1. (Saul, 2018)

The strain responsible for the 1918 flu pandemic was a type-A virus, which are most linked to deadly pandemics. A form of H1N1, it seems to have picked up a mutation that made it unusually lethal, though scientists studying it still can’t discern what made this particular strain so deadly. The virus was most likely circulating in pigs and some humans for anywhere from two to fifteen years before the pandemic began.

The # of people killed by the 1918 flu pandemic

Over the span of 18 months at least one-third of the world’s population was infected. Estimates of the number of people killed vary from as low as 20 million to as high as 100 million, but most experts put the death toll at 50 million or more. If the upper end of the estimate is accurate, then the 1918 pandemic will have killed more people than both world wars put together.

“In the United States stories abounded of people waking up sick and dying on their way to work,” writes Toby Saul (2018). Sometimes entire families would be wiped out while neighbors recovered, suggesting a certain genetic profile may have put certain people at more risk.

The 1918 strain that caused this pandemic is estimated to have killed anywhere from 1% to 3% of those it infected, showing that it doesn’t take a high lethality rate to cause a lot of problems. This compares to a 0.03% lethality rate for the 2009 H1N1 strain that got everyone’s attention, and 40% for the rare H7N9 strain currently circulating in China. (Yong, 2018)