The practice started in China during the Manchurian plague outbreak of 1910, as this podcast describes. The plague was a particularly nasty one, killing 95% of the people who contracted it. A Chinese doctor was the first one to propose the idea of wearing masks in public to prevent the spread of the disease. He was laughed at, until a French doctor in the territory died from the disease. After that, everyone wore masks, it made international news, and the outbreak ended in 7 months. When the 1918 Influenza outbreak occurred, people had to be persuaded to wear masks again, and when the Communists took over China, they ran numerous propaganda campaigns urging people to wear masks whenever they felt a little unwell.