Cancer
Before the 19th century (the 1800s), using surgery to treat cancer usually had bad results. Doctors did not understand how important hygiene (or keeping things clean) is for preventing disease, especially after surgery. Because things were not kept clean during or after surgery, patients often got infections and died. For example, one well-known Scottish surgeon, Alexander Monro, kept records and found that 58 patients out of every 60 who had surgery for breast tumors died within the next two years.