I think her Cancer was genetic, her sister died from the same type of Cancer and her mother died of cancer..
Everyone has cancer cells and stress and a family history can activiaye them, especially if you have a weak immune system...
There are genetic predispositions and you are right everyone has mutant cells. However there are a lot of ways that people could be cautioned concerning the other causes which includes a number of things that make people uncomfortable.
I know that there is a predilection for certain cancers in some families, but Wikipedia says that Farrah's sister, Diane Fawcett Walls, died of lung cancer. It also mentions that her mother died in 2005, but doesn't give a cause. She did however make it to age 91. To say that cancer is terrible is truly an understatement and my heart and prayers go out to all who have been touched by it.