Learning by Teaching: The 1918 Spanish Flu

In light of the events in Japan, this may be the most under-learned natural disaster in history:

The pandemic lasted from June 1917 to December 1920, spreading even to the Arctic and remote Pacific islands. Between 50 and 100 million died, making it one of the deadliest natural disasters in human history. Even using the lower estimate of 50 million people, 3% of the world's population (1.8 billion at the time), died of the disease. Some 500 million, or 28% (≈1/4) were infected.

Compare that to the 9 million that died in WWI, the largest war in Western history at that time.