actually, the body attacks its own unhealthy cells all the time in order to keep the body functioning properly but that is the reason why cancer is a problem. If for some reason the person contracts a disease which weakens the immune system (or other ailments of similar accord such as diet, exercise, etc), then the immune system cannot cope with the new problem at hand.
i'll look up the rate cancer cells multiply: and that wasn't helpful.
you are correct that the body does not attack the unhealthy cells, but that becomes the problem. It does attack them, but at the rate cancer cells grow, it would be like diverting every white blood cell in the body to deal with one infection, which is impossible
so, it is very similar to our problems with league at the moment, small problems continue to grow out of hand and then a major problem arises when the body itself can no longer handle the infection
cancer is very much like a virus but the only distinction between it and a virus is that it is the body's own cells that are the problem, which makes the problem worse.
Viruses work by injecting parts of their own genetic code into a cell and this makes that cell reproduce with a copy of the virus in each new cell. Cancer cells are the body's own cells suffering from a mutation that can go undetected but causes problems with the healthy cells due to a lack of food and space for the body to sustain that part of itself. Effectively, cancer causes you to rot from the inside out, just like a virus, but unlike most viruses, Cancer cells divide endlessly until the body dies whereas viruses divide to reproduce and move to new regions to infect more host bodies.