The thought of having cancer can be scary. But what exactly is cancer? And what does it do?

Cancer refers to any one of a large number of diseases characterized by the development of abnormal cells that divide uncontrollably and have the ability to infiltrate and destroy normal body tissue. Cancer can spread its abnormal cells throughout your body.

Being diagnosed with cancer can be frightening. But understanding what's going on inside your body can help you be aware of what's causing your feelings of anxiety. Knowing more about cancer may also help you feel more in control of your disease.

Cancer is caused by damage (mutations) in your DNA. Your DNA is like a set of instructions for your cells, telling them how to grow and divide. When a mutation occurs in your DNA, normal cells will repair the mutation or simply die. In cancer, your cells continue living with this mutation. As a result, they grow and divide in chaotic fashion.

Mutations in your DNA can be caused by:

Your own habits
Certain lifestyle choices are known to cause cancer. Smoking, drinking more than one drink a day (for women) or two drinks a day (for men), being overweight, damage to skin unprotected by sunscreen, and unsafe sexual behaviors can lead to the mutations that cause cancer. You can break these habits to lower your risk of cancer — though some habits are easier to break than others.

Your environment
The environment around you may contain harmful chemicals that can cause mutations in your genes. Even if you don't smoke, you might breathe secondhand smoke if you go places where people are smoking or you live with someone who smokes. Chemicals in your home or work environment, such as asbestos and benzene, can also cause cancer.

Your family history
Some mutations in your DNA can be traced back to your parents. If cancer is common in your family, it's possible that mutations are being passed from one generation to the next. You might be a candidate for genetic screening to see whether you have mutations that might increase your risk of cancer. Keep in mind that having an inherited genetic mutation doesn't necessarily mean you'll get cancer.

Chronic conditions
Some chronic health conditions, such as ulcerative colitis, can develop into cancer. Talk to your doctor about your risk.

Much is still unknown about cancer. Some people with many risk factors don't develop cancer, and some people with no apparent risk factors develop cancer nonetheless.

Cancer cells grow in an uncontrolled manner. One malignant cell becomes two, two become four, four become eight, and so on, until a mass of cells (a tumor) is created. The tumor interferes with the normal functioning of healthy tissue and can spread to other parts of your body.

Not all tumors are cancerous, and not all cancers form tumors. For example, leukemia is a cancer that involves blood, bone marrow, the lymphatic system and the spleen but doesn't form a single mass or tumor.

Cancer invades and destroys normal tissue. It can also produce chemicals that interfere with body functions. For instance, some lung cancers secrete chemicals that alter the levels of calcium in your blood, affecting nerves and muscles and causing weakness and dizziness.

Cancer can also spread (metastasize) and invade healthy tissue in other areas of your body.

Cancer can take years to develop. By the time a cancerous mass is detected, it's likely that 100 million to 1 billion cancer cells are present, and the original cancer cell may have been dividing for five years or more.

The only way to diagnose cancer is to examine the cells under a microscope. Some imaging tests, such as computerized tomography (CT) scan or mammography, can indicate the possible presence of cancer, but cancer can be definitively diagnosed only by looking closely at cancer cells.

Your doctor uses a surgical process called a biopsy to get a sample of suspect tissue.

Under the microscope, normal cells look uniform, with similar sizes and orderly organization. Cancer cells look less orderly, with varying sizes and without apparent organization.

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Cancerous and noncancerous cells The image of the normal colon tissue, at left, shows well-formed ... << ENLARGE

Cancer doesn't discriminate when it comes to race, sex or age — anyone can get cancer. The American Cancer Society (ACS) estimates that half the men and one-third of the women in the United States will develop cancer during their lifetimes.

The ACS estimates that more than 1.3 million Americans will be diagnosed with cancer in 2004, and about 564,000 people will die of the disease. The types of cancer you're at greatest risk for varies based on your sex, age, where you live and habits such as diet and exercise.

Until recently, cancer was incurable. But today, cancer is no longer an automatic death sentence. More than half the people with a diagnosis of cancer survive five years or more after their diagnosis.

One key to survival is early detection of cancer. Talk to your doctor about cancer screenings, if you aren't already on a regular schedule of screenings. The tests you should have depend on your sex, age and family history of cancer.

As more is known about cancer, researchers are discovering ways you can protect yourself from cancer before it starts to form. These include:

• Don't smoke or use tobacco products.
• Limit your exposure to sun and use sunscreen.
• Drink alcohol only in moderation, if at all.
• Exercise and stay active.
• Maintain a healthy weight.
• Be aware of risk factors in your home and workplace, and take steps to reduce your exposure to these substances.

Talk to your doctor about your cancer risk. Routine physical examinations can help you and your doctor identify your behaviors and other factors that might increase your risk of cancer.