Plague is transmitted to humans via flea bites, handling of an infected animal or through the air in close quarters when a plague-infected person coughs (the CDC says the latter hasn't happened in the United States since 1924). The two most infamous plague epidemics both originated in China, according to the CDC: the "Black Death," which began in 1334 and spread along trade routes to Europe, which saw an estimated 60% of its population wiped out; and a "Modern Plague" that began in 1860 and caused an estimated 10 million deaths as it spread to port cities across the world.