Today we know that cancer is not usually caused by any one factor. People who are genetically ready to develop certain types of cancer are at high risk of developing cancer when they are exposed to certain factors in the environment (carcinogens). The match between susceptibility and time, duration and dose of carcinogen exposure leads to cancer characteristics: where, to whom and when. For many cancers, we know more about genetic susceptibility, while for others we know more about involved carcinogens.

Genetic predisposition: The fact that cancer can “run in families” has been recognized for over a century. Inherited cancer syndromes, usually involving mutations in tumor suppressors or DNA repair genes, may account for up to 4% of all cancers. Clear inheritance of cancer occurs in rare types of familial cancer syndromes. An example is Retinoblastoma (cancer of the retina) which usually affects young children. When a child inherits the abnormal gene from one of the parents, the disease will affect both eyes at a young age. It seems that all cells in the retina carry this genetic abnormality, making the likelihood of cancer very high. Inheritance is not very clear in the most common types of cancer.

Carcinogens: A long list of factors that can lead to cancer exists, these include:

• Smoking: In addition to lung cancer, tobacco consumption causes tumors of the larynx, pancreas, kidney, and bladder and, in conjunction with alcohol drinking, a high incidence of carcinomas of the oral cavity and the esophagus. In most developed countries, tobacco accounts for as much as 30% of all malignant tumors.
• Heavy alcohol drinking can cause cancer of the oral cavity, pharynx, larynx, esophagus, and liver, and may increase the risk of breast and colorectal cancers.
• Environmental factors such as: pollution of air, water or soil is estimated to account for 1-4% of cancers. Indoor carcinogens include tobacco smoke. High levels of arsenic and chlorination byproducts in some countries reach dangerous levels that may be associated with cancer.
• Drugs: Drugs used to treat cancer (chemotherapy) can cause cancer themselves. Another example is drugs with hormonal activity. Hormone replacement therapy is associated with increases in the risk of breast and endometrial cancers although, they may relieve other health problems associated with menopause.
• Chronic infections: Hepatitis B infection is associated with liver cancer. The human papillomavirus is associated with cancer of cervix and uteri.