Cancer is characterized by the uncontrolled growth of abnormal cells. In a healthy adult, millions of cells grow, divide and die each day to replace worn-out or dying cells or to repair injury. This process is tightly controlled. Should the genetic material of a particular cell become damaged — as a result of smoking, pollutants in the environment or simply bad luck — that cell can begin to multiply uncontrollably. Over time, these cancer cells can accumulate and form tumors, which can harm the body in a number of ways.

Tumors can invade and destroy nearby tissue. Some tumors secrete hormones or enzymes that disrupt the body's normal functions. As tumors grow, they develop networks of blood vessels to provide them with nourishment. They begin to rob healthy cells of essential nutrients. Eventually, cells may break away from the tumor and spread to other parts of the body, where they establish tumor colonies. This spread of cancer is called metastasis and cancer that has spread is said to have metastasized.

The Genetic Link

Virtually every cancer is caused by alterations (mutations) of DNA, the genetic material that controls how cells behave. When DNA is damaged, genes called oncogenes can become activated. Oncogenes instruct a cell to keep dividing. DNA damage also can inactivate genes called tumor-suppressor genes, which keep a cell from dividing. Most cancers begin because oncogenes are activated, and tumor-suppressor genes are inactivated.

Most genetic damage is believed to be caused by environmental factors, such as exposure to chemicals, radiation, smoke and pollution, or viruses, such as hepatitis B. In addition, cell mutations may occur by mistake as cells divide. These mistakes also can be inherited, which is why many cancers run in families.

A Preventable Disease

As researchers have gained a better understanding of the risk factors associated with various cancers, they have come to believe that the majority of cancers can be prevented. You can reduce your risk of cancer by limiting your exposure to substances that are known to promote cancer. Cigarette smoke, for example, vastly increases the risk of lung cancer. Cigarette smoke also is believed to increase the risk of other cancers, such as pancreatic, bladder and cervical cancer.

Scientists have made great strides in understanding and treating cancer. Even when cancer can't be prevented, advances in screening and early detection have made it possible to diagnose cancers at the earliest possible stage. As a general rule, the smaller and more confined the tumor is at the time of diagnosis, the better the chance of achieving a permanent cure.