The hits may come from chemical or foreign substances that cause cancer, called carcinogens. 
These initiate the cancer process. Or the hits may be promoters that accelerate the growth of 
abnormal cells . What is critical is the number and types of hits, their frequency and their intensity.

Initiators include: 
• Tobacco and tobacco smoke carcinogens. Lung cancer was a rare disease before cigarette smoking became
  widespread.
• X-rays. It is well known that there is an increased incidence of leukemia among atomic bomb survivors. This
  same increase has been noted in radiologists, the doctors who specialize in the use of x-rays. It should be
  pointed out that the average diagnostic use of x-rays does not increase the chance of getting cancer enough to
  rule out their use as a valuable health care tool. The risk of not having an x-ray may far outweigh the risk of
  cancer being caused by the use of x-rays for diagnosis. Used properly, x-rays are of great benefit in finding
  cancer at an early, curable stage.
• Certain hormones and drugs, such as DES , some estrogens (female hormones) and some immunosuppressive
  drugs.
• Excessive exposure to sunlight. 
• Industrial agents or toxic substances in the environment, such as asbestos, coal tar products, benzene,
  cadmium, uranium and nickel.