Many chemical additives in cigarettes are considered to be carcinogenic, which greatly increases the risk of many types of cancer in people who smoke. The Epstein-Barr virus is an example of a viral carcinogen, as it increases the risk of lymphoma. Radon is a significant environmental exposure that can lead to lung cancer.
Exposure to a carcinogen doesn't mean you will absolutely develop cancer. There are many factors involved, such as the type of carcinogen, how long you were exposed, and the frequency of exposure.