So I took a class about oncogenesis (cancer growth), and we had been talking about possible causes for gene mutations. That is when I realized that nearly anything that is supposedly good for you (sunlight, legumes, sunscreen, wheat, etc) actually can cause gene mutations. And you can't simply say "well I'm just going to avoid these things altogether. That'll help me avoid cancer" because if you avoid sunlight altogether, you can become vitamin D deficient, which a lot of the population already is, or you can miss out on the benefits that certain food bring you. And realistically, avoid everything that could cause you to have oncogenic gene mutations would be basically to avoid life. Everything causes some kind of mutation. some mutations you are born with. I believe that basically, we all just need to realize that cancer is a likely thing to happen, and try to be as aware of it as possible to be able to catch it before it kills us.
About the contagious cancer in tasmanian devils : It's caused by a virus which just so happen to insert its genetic code into a part of the host's genome which is too close to an important functional gene.