Cancer is the uncontrolled growth of abnormal cells in the body. Cancerous cells are also called malignant cells.

Cells are the building blocks of living things. Cancer grows out of normal cells in the body. Normal cells multiply when the body needs them, and die when the body doesn't need them. Cancer appears to occur when the growth of cells in the body is out of control and cells divide too quickly. It can also occur when cells “forget” how to die.

There are many different kinds of cancers. Cancer can develop in almost any organ or tissue, such as the lung, colon, breast, skin, bones, or nerve tissue.

There are many causes of cancers, including:

• Benzene and other chemicals
• Certain poisonous mushrooms and a type of poison that can grow on peanut plants (aflatoxins)
• Certain viruses
• Radiation
• Sunlight
• Tobacco

However, the cause of many cancers remains unknown.

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

The three most common cancers in men in the United States are:

• Prostate cancer
• Lung cancer
• Colon cancer

In women in the U.S., the three most common cancers are:

• Breast cancer
• Colon cancer
• Lung cancer

Some cancers are more common in certain parts of the world. For example, in Japan, there are many cases of gastric cancer, but in the U.S. this type of cancer is pretty rare. Differences in diet may play a role.

Some other types of cancers include:

• Brain cancer
• Cervical cancer
• Hodgkin's lymphoma
• Kidney cancer
• Leukemia
• Liver cancer
• Non-Hodgkin's lymphoma
• Ovarian cancer
• Skin cancer
• Testicular cancer
• Thyroid cancer
• Uterine cancer

Symptoms of cancer depend on the type and location of the tumor. For example, lung cancer can cause coughing, shortness of breath, or chest pain. Colon cancer often causes diarrhea, constipation, and blood in the stool.

Some cancers may not have any symptoms at all. In certain cancers, such as gallbladder cancer, symptoms often do not start until the disease has reached an advanced stage.

The following symptoms can occur with most cancers:

• Chills
• Fatigue
• Fever
• Loss of appetite
• Malaise
• Night sweats
• Weight loss

Like symptoms, the signs of cancer vary based on the type and location of the tumor. Common tests include the following:

• Biopsy of the tumor
• Blood chemistries
• Bone marrow biopsy (for lymphoma or leukemia)
• Chest x-ray
• Complete blood count (CBC)
• CT scan

Most cancers are diagnosed by biopsy. Depending on the location of the tumor, the biopsy may be a simple procedure or a serious operation. Most patients with cancer have CT scans to determine the exact location and size of the tumor or tumors.

A cancer diagnosis is difficult to cope with. It is important, however, that you discuss the type, size, and location of the cancer with your doctor when you are diagnosed. You also will want to ask about treatment options, along with their benefits and risks.

It's a good idea to have someone with you at the doctor's office to help you get through the diagnosis. If you have trouble asking questions after hearing about your diagnosis, the person you bring with you can ask them for you.

Treatment also varies based on the type of cancer and its stage. The stage of a cancer refers to how much it has grown and whether the tumor has spread from its original location.

• If the cancer is confined to one location and has not spread, the most common goals for treatment are surgery and cure. This is often the case with skin cancers, as well as cancers of the lung, breast, and colon.
• If the tumor has spread to local lymph nodes only, sometimes these can also be removed.
• If surgery cannot remove all of the cancer, the options for treatment include radiation, chemotherapy, or both. Some cancers require a combination of surgery, radiation, and chemotherapy.

Although treatment for cancer can be difficult, there are many ways to keep up your strength.

If you have radiation treatment, know that:

• Radiation treatment is painless.
• Treatment is usually scheduled every weekday.
• You should allow 30 minutes for each treatment session, although the treatment itself usually takes only a few minutes.
• You should get plenty of rest and eat a well-balanced diet during the course of your radiation therapy.
• Skin in the treated area may become sensitive and easily irritated.
• Side effects of radiation treatment are usually temporary. They vary depending on the area of the body that is being treated.

If you are going through chemotherapy, you should eat right. Chemotherapy causes your immune system to weaken, so you should avoid people with colds or the flu. You should also get plenty of rest, and don't feel as though you have to accomplish tasks all at once.

It will help you to talk with family, friends, or a support group about your feelings. Work with your health care providers throughout your treatment. Helping yourself can make you feel more in control.

A cancer diagnosis often causes a lot of anxiety and can affect your entire quality of life. Several support groups for cancer patients can help you cope.

The outlook depends on the type of cancer. Even among people with one type of cancer, the outcome varies depending on the stage of the tumor when they are diagnosed.

Some cancers can be cured. Some cancers that are not curable can still be treated well. And some patients can live for many years with their cancer. Other tumors are quickly life-threatening.

One complication is that the cancer may spread. Other complications vary with the type and stage of the tumor.

Contact your health care provider if you develop symptoms of cancer.

One of the best ways to prevent cancer is to not smoke or chew tobacco. Many cancers can be prevented by avoiding risk factors such as excessive exposure to sunlight and heavy drinking.

Cancer screenings, such as mammography and breast examination for breast cancer and colonoscopy for colon cancer, may help catch these cancers at their early stages when they are most treatable. Some people at high risk for developing certain cancers can take medication to reduce their risk.