Cancer
In the 19th century, surgical hygiene got better because of asepsis. Doctors realized that dirtiness and germs cause infections, so they started to keep things cleaner and do things to kill germs in order to prevent their patients from getting infections. It became more common for people to survive after having surgery. Surgical removal of the tumor (taking the tumor out of the body by doing surgery) became the first-choice treatment for cancer. For this kind of treatment to work, the surgeon doing the operation had to be very good at removing tumors. (This meant that even if people had the same kind of cancer, they could get very different results, with some getting good treatment that worked and others getting treatment that did not work, because of differences in how good different surgeons were.)