Because there were only a few ways of studying how the body worked, early doctors did not know how to cure sickness. When a plague started, they could not really know what to do for their patients except making them a little less painful. One of their mistakes was that they thought the "air" carried the disease (which it sometimes did), or that sickness was caused by "flying venom": so, they covered their mouths or burned aromatic leaves (leaves that smelled good), but did not wash their hands after seeing a sick friend or family member. When William Harvey practiced medicine in 1618, only a few of his friends were trying to find out how the body worked, and only a few doctors believed Harvey when he said that the blood was pumped by the heart. Most people thought he was a quack (fake or bad doctor). Slowly they began to see his findings were real.