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Cancer is not just one disease but many diseases. There are more than 100 different types of cancer. Most cancers are named for the organ or type of cell in which they start - for example, cancer that begins in the colon is called colon cancer; cancer that begins in basal cells of the skin is called basal cell carcinoma.
Cancer is a term used for diseases in which abnormal cells divide without control and are able to invade other tissues. Cancer cells can spread to other parts of the body through the blood and lymph systems.
Cancer types can be grouped into broader categories. The main categories of cancer include:
All cancers begin in cells, the body's basic unit of life. These cells grow and divide in a controlled way to produce more cells as they are needed to keep the body healthy. When cells become old or damaged, they die and are replaced with new cells. However, sometimes this orderly process goes wrong. The genetic material (DNA) of a cell can become damaged or changed, producing mutations that affect normal cell growth and division. When this happens, cells do not die when they should and new cells form when the body does not need them. The extra cells may form a mass of tissue called a tumor.
Not all tumors are cancerous; tumors can be benign or malignant.
Some cancers do not form tumors. For example, leukemia is a cancer of the bone marrow and blood.
Source: National Cancer Institute - http://www.cancer.gov/cancertopics/cancerlibrary/what-is-cancer