Cancer Cells

Cancer cells are the result of mutations that disable the mechanisms that keep cell growth in check. Those mutations come from incorrect repairs to cell DNA, and those errors happen more frequently the more repairs take place. If the body is forced to do repairs often and faster (e.g. for smokers), it will make more mistakes.

The human body already has a number of ways to deal with cancer though. Cancer cells are produced in the body all the time but those cells have mechanisms in place to kill themselves if they become cancerous or they are obliterated by the immune system. It needs to get past both of those things and start reproducing out of control to become an issue.