Killed 50 to 100 million people worldwide in less than 2 years.
In 1918 and 1919, the Spanish Flu pandemic killed more people than Hitler, nuclear weapons and all the terrorists of history combined. (A pandemic is an epidemic that breaks out on a global scale.) Spanish influenza was a more severe version of your typical flu, with the usual sore throat, headaches and fever. However, in many patients, the disease quickly progressed to something much worse than the sniffles. Extreme chills and fatigue were often accompanied by fluid in the lungs. One doctor treating the infected described a grim scene: "The faces wear a bluish cast; a cough brings up the blood-stained sputum. In the morning, the dead bodies are stacked about the morgue like cordwood."
If the flu passed the stage of being a minor inconvenience, the patient was usually doomed. There is no cure for the influenza virus, even today. All doctors could do was try to make the patients comfortable, which was a good trick since their lungs filled with fluid and they were wracked with unbearable coughing. The "bluish cast" of victims' faces eventually turned brown or purple and their feet turned black. The lucky ones simply drowned in their own lungs. The unlucky ones developed bacterial pneumonia as an agonizing secondary infection. Since antibiotics hadn't been invented yet, this too was essentially untreatable. The pandemic came and went like a flash. Between the speed of the outbreak and military censorship of the news during World War I, hardly anyone in the United States knew that a quarter of the nation's population -- and a billion people worldwide -- had been infected with the deadly disease. More than half a million died in the U.S. alone; worldwide, more than 50 million.