Cancer
Hippocrates and other doctors at that time treated people based on the humor theory. This theory said that there were four types of fluid in the body (black, yellow bile, blood, and phlegm). Doctors tried to figure out whether these four "humors" (or body fluids) were in balance. They would then use treatments like blood-letting (cutting the patient and letting him bleed so that he would lose blood); laxatives (giving the patient foods or herbs to make him go to the bathroom), and/or changing the patient's diet. The doctors thought that these treatments would work to get the patient's four humors back into the right balance. The humor theory treatment was popular until the 19th century (the 1800s), when cells were discovered. By this time, people had realized that cancer can happen anywhere in the body.