Cancer is the abnormal growth, reproduction and spread of body cells. The human body contains trillions of cells grouped to make tissues such as muscles, bones and skin.
Most normal cells grow, reproduce and die in response to signals inside and outside the body.
If these processes occur in a balanced and orderly way, the body remains healthy and functions normally. Problems can begin when normal cells mutate, or change, into cancer cells. A normal cell can become a cancer cell for no apparent reason or because of repeated or heavy exposure to a carcinogenic, or cancer-causing, substance such as tobacco or alcohol. Both the behavior and appearance of cancer cells differ from normal cells. Mutation occurs in the cell's DNA, or genetic material.