What is the worst disease in history?

the worst disease in history was the Bubonic plague

Explanation:

Death Toll: 75 - 200 million

From 1346 to 1353 an outbreak of the Plague ravaged Europe, Africa, and Asia, with an estimated death toll between 75 and 200 million people. Thought to have originated in Asia, the Plague most likely jumped continents via the fleas living on the rats that so frequently lived aboard merchant ships. Ports being major urban centers at the time, were the perfect breeding ground for the rats and fleas, and thus the insidious bacterium flourished, devastating three continents in its wake.

The Bubonic Plague was the worst disease in medical history. It killed over 17 million people.