Risk factors

Doctors often cannot explain why one person develops cancer and another does not. But research shows that certain risk factors increase the chance that a person will develop cancer. These are the most common risk factors for cancer:

Growing older

Tobacco

Sunlight

Ionizing radiation

Certain chemicals and other substances

Some viruses and bacteria

Certain hormones

Family history of cancer

Alcohol

Poor diet, lack of physical activity, or being overweight
http://www.medicinenet.com/cancer_causes/article.htm