Named for Roman Emperor Marcus Aurelius Antoninus, who ruled during the outbreak along with co-regent Lucius Verus, the outbreak began in 165 and lasted until 180. An estimated five million people died from what is now thought to have been smallpox. It's believed to have begun in the Mesopotamian city of Seleucia (in modern-day Iraq) and spread to Rome by soldiers returning from the city's siege. At one point during the extended pandemic an estimated 2,000 Romans died each day.