From January 1918 to December 1920, a deadly influenza outbreak infected 500 million people worldwide. It is estimated that 50 to 100 million people died from the virus, in other words, up to 5% of the planet's population. It killed more people than any other disease in recorded history, more than the total number of deaths in WWI.
The Spanish flu strain killed its victims in a way never seen before. The United States is full of stories of people getting sick and dying on the way to work.
The symptoms were gruesome: Victims would have a fever and have trouble breathing. The lack of oxygen meant that their faces appeared to be painted blue.
The hemorrhage filled the lungs with blood and caused catastrophic vomiting and nose bleeds, with victims drowning in their own fluids. Unlike so many types of influenza before it, the Spanish flu strikes not only the very young and the very old, but also healthy adults between the ages of 20 and 40.
The major factor in the spread of the virus, of course, was the then international conflict which was in its final stages. Epidemiologists still dispute the exact origin of the virus, but there is some consensus that it was the result of a genetic mutation that probably occurred in China.