One of the things that makes cancer insidious is that it is essentially you. Most diseases are caused by some foreign agent. Some bacteria or virus that invades your body and shuts down or takes over some critical function.

Cancer can be triggered by a foreign agent, but it itself is not a foreign agent. It is your own body's natural processes gone haywire. That makes differentiating cancer cells from other cells more difficult. Chemotherapy works by essentially killing your most active cells. The cells that are constantly reproducing. Those are your hair follicles, your stomach lining, your cancer, etc. It's why the side effects include hair loss and nausea. It's a chemical shotgun blasted into your body with the hopes that one of the casualties is the cancer.