Cancer is named for the constellation of the crab because of the crab-like appearances of tumors. In more general terms, most people did not have access to anyone trained in any sort of medicine, and often the practitioners were just basing diagnosis and treatment on the superstitions of the day. So if the local witchy-woman (like my granny) said you had the pox, you had the pox. If the village wise one said you had cancer, you had cancer. In the long run it made little difference. If you were sick, you died.

There are some truly gruesome illustrations of various poxes, cancers, growths, and similar calamities available online. Not for the weak of stomach.