Cancer
In the 16th and 17th centuries (the 1500s and 1600s), doctors started to be allowed to dissect bodies (or cut them open after death) in order to figure out the cause of death. Around this time, there were many different ideas about what caused cancer. The German professor Wilhelm Fabry believed that breast cancer was caused by a clot of milk in the part of a woman's breast that produces milk. The Dutch professor Francois de la Boe Sylvius believed that all disease was caused by chemical processes. He thought that cancer, in particular, was caused by acidic lymph. Nicolaes Tulp, who lived at the same time as Sylvius, believed that cancer was a poison that slowly spreads and was contagious.