China's Yunnan province was the setting for the third and last great plague pandemic, which began in the 1850s. The infectious fever wreaked havoc on Chinese citizens, killing tens of thousands, and by the late 19th century it had made its way to Hong Kong and Guangzhou, which proved devastating. The world was more interconnected than it had ever been, and both cities were major coastal shipping hubs. This allowed the disease to spread throughout the world — to Bombay, Cape Town, Guayaquil in Ecuador, San Francisco and Pensacola, Fla. In the end, the plague killed some 12 million people. However, it occurred at a time when scientific understanding of diseases was developing, giving doctors and scientists a perfect empirical test case for germ theory and new medicines. This has been a large part of why we haven't seen a fourth pandemic.