Although the Spanish influenza originated in China, the first large number of casualties were in Spain in May of 1918. The first wave swept through American military training camps in March of 1918, but didn't become epidemic until the winter of that same year.
The Spanish influenza spread along shipping lines and trade routes. Outbreaks swept throughout Europe, North America, Asia, Brazil, the South Pacific and Africa.
The Spanish influenza pandemic killed between 20 and 40 million individuals, which was higher than the death toll credited to World War I. More people died from Spanish influenza in one year than in all four years of the Bubonic Plague (1347 to 1351).
Approximately 675,000 Americans died of Spanish influenza during the pandemic. The average life span for those living in the United States was decreased by 10 years.
Many individuals thought the deadly Spanish flu was the result of German biological warfare. Others believed it was a side effect of the mustard gases used during the war.
The Spanish influenza primarily killed individuals between the ages of 20 and 40. This pattern of morbidity is unusual for influenza outbreaks, which usually kill young children and the elderly.