Cancer is a disease in which cells multiply wildly, destroy healthy tissue, and endanger life. About 100 kinds of cancer attack people. Cancer strikes people of all ages but is most common in the middle-aged and the elderly. The disease is a leading cause of death in many countries. Cancer occurs about equally in males and females. The disease can attack any part of the body and may spread to other parts. Cancer occurs in most other animals as well as in people.
The cells of all living things contain genes that direct cell growth and reproduction. Cancer arises when these genes become damaged and cells multiply without control. The damaged genes may instruct the cell to divide in an abnormal fashion. Other genes may lose the ability to instruct the cell that it can no longer function and must die. Some damaged genes are changed in ways that enable cancer cells to invade healthy tissue, grow new blood vessels, or spread to other parts of the body.
Some types of abnormalities in genes are inherited. Substances in the environment can also damage genes. A substance that damages genes in a way that can lead to cancer is called a carcinogen. Many substances found in nature, both natural and artificial, are carcinogens.
Without proper treatment, most kinds of cancer are fatal. However, methods of diagnosing and treating the disease have improved greatly. About half of all cancer patients now survive at least five years after treatment. People who remain free of cancer during that time have a good chance of remaining permanently free of the disease. More research is needed to find even better methods of curing cancer. Because cancer affects so many people, many countries spend public money to fund cancer research.