The bubonic plague originated in the Gobi Desert in the 6th century and spread along trade routes to Europe, Asia, Africa and other parts of the world. It caused pandemics in the 6th and 14th centuries, killing over 200 million people. The disease was carried by fleas that lived on rats and was easily transmitted between humans. Symptoms included painful swellings, fever, vomiting and bleeding in the lungs. Treatment methods included quarantine, herbal remedies and leeches.