Cancer involves the uncontrolled growth of abnormal cells that have mutated from normal tissues. This growth can kill when these cells prevent normal function of vital organs or spread throughout the body, damaging essential systems.

There are at least 200 different kinds of cancers. They can develop in almost any organ, fluid, or tissue. Some cancers affect blood cells, some affect skin; others affect bone, muscle, or nerve tissue.

In general, cancer appears to be caused by abnormal regulation of cell growth. Typically, the growth of cells in the body is strictly controlled -- new cells are made as needed to replace older ones or to perform needed functions. Cells die when they become damaged and/or are no longer needed. If the balance of cell growth and death is disturbed, cancer may occur.

Problems in the regulation of cell growth can be caused by abnormalities of the immune system, which normally would detect and stop aberrant growth. Other potential causes of cancer include the following

• Radiation
• Sunlight
• Tobacco
• Certain viruses
• Benzene
• Certain poisonous mushrooms and aflatoxins (a poison produced by organisms that can grow on peanut plants)

The three most common cancers in men in the U.S. are prostate cancer, lung cancer, and colon cancer. In women here, the three most frequently occurring cancers are breast cancer, lung cancer, and colon cancer.

The most common cause of cancer-related death is lung cancer.

Certain cancers are more common in particular geographic regions. For example, in Japan, there are many cases of gastric cancer while in the US this type of cancer is relatively rare. Dietary differences may account for the variance.