This may be a stupid question but
Faulty (mutated) tumor suppressor genes can be inherited. Oncogenes are NOT inherited but, defective tumor suppressor genes are. This is often confused with "inheriting cancer".
What causes mutations to genes? Carcinogens is the term for anything take has the potential to initiate cancer. All mutagens are carcinogens but, not all carcinogens are mutagens. Mutagens are anything that can mutate a cell (chemicals, radiation, tobacco, etc.). Some examples of carcinogens that are not mutagens are alcohol and abestos. Alcohol kills cells and the killed cancer cause cells to divide. Anytime cells divide is an opportunity for genetic mutations to be introduced and propagate.
Some viruses have also been identified to each cause cancer or have a strong association with cancer including HPV, AIDS, Hepatitis, ATLV and, EBV. Again, these viruses contain oncogenes.
We are constantly exposed to carcinogens. We can limit our exposure but, never eliminate it (viruses, pollution, sunlight, etc). Mutations are bound to happen.
Cancer may actually start and end before any detection if the immune system recognizes the (pre)cancerous cell and kills it. As we age, our immune system becomes less effective.
The longer you live the more chances cancer will eventually happen. This is why cancer is primarily a disease of the old with most patients diagnosed over 60 years of age.
All mammal get cancer thought out their life but for cancer to progress both factor A and factor B have to work together for the cells to mutate or divide and grow.