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Cancer occurs when a single cell starts to divide repeatedly, producing abnormal copies of itself, rather than dividing ocassionally just to replace worn out cells. If the imune system does not destro these cells , they continue to to reproduce and invade surrounding tissue. Cancer cells can spread to other parts of the body through the blood and lymph  systems. Cancer is not just one disease but many diseases. Most cancers are named for the organ or type of cell in which they start: for example, cancer that begins in the breast is called breast cancer. Although there are more than 100 different types of cancer, cancers can be broadly grouped into four types, depending on which tissues they come from:

• Carcinomas: arise from the cells that cover external and internal body surfaces. For example, lung, breast, and colon.
• Sarcomas: arise from cells found in the supporting tissues of the body such as bone, cartilage, fat, connective tissue and muscle.
• Lymphomas: arise in the lymph nodes and tissues of the body's immune system.
• Leukaemias: arise from immature blood cells that grow in the bone marrow and tend to accumulate in large numbers in the bloodstream.