iwbus, the answers already posted are very well put. Cancer can also attack weakened areas in the body. In my case, I got bone cancer where I had injured and weakened the bone as a child. It spread from the bone to other areas and I ended up having years of treatment. The good news is though, that one should never give up if diagnosed with cancer. I was told I had no chance of survival and here I am! I have been cancer free for over 17 years now!! Sadly my son died of brain cancer. This attacked where he had hit his head on some gym equipment in school years earlier. His sports teacher had omitted to put the rubber mats out and he fell and hit his head on the iron feet of a piece of equipment. That is where the brain tumour eventually grew. What sparked its growth, according to leading specialists, was smoking. The smoking killed off brain cells in the weakened area and so the cancer had this weak area to attack. What we must not forget is that a great number of cancer patients DO survive and live a long and healthy life. We don't hear about the survivors so often, as it is more sensational for the press to report on the negative side of things. I have seen both ... and the negative could not have been more devastating. I will never stop missing my beautiful son, but he lives in my heart now and we laugh and smile a lot together.