Body Trivia : the deadliest natural disasters

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What is the epidemic that caused the deadliest natural disasters in human history?


It is influenza pandemic in January 1918 – December 1920. 

 It infected 500 million people across the world and killed 50 to 100 million of them—three to five percent of the world's population. 
This disease was first observed at Kansas state in USA in January 1918. At that time, the report was controlled because it was during World War I. 
In neutral Spain papers were free to report the epidemic's effects, creating a false impression of Spain as especially hard hit. Thus the pandemic's nickname Spanish flu.　

One of the causes of the expansion of the infection was a vaccination.