Infectious disease has been a formidable force in shaping human history. In the times past, most people died from two causes: violence and infectious disease, with deaths from infectious disease being many times more common. Bubonic plague killed between a third and a half of the population of Europe in the Middle Ages, thus changing the course of Europe and the world forever. Smallpox killed half a billion people in the 20th century alone before being finally eradicated in 1982.