In a healthy body, the cells in your body normally divide and grow to replace dying or dead cells. This process is controlled by the genes in the cell (your DNA). Sometimes, the genes change (mutate) and the cells start to divide and grow out of control. These cells are referred to as being cancerous or malignant.

Cancer is not one disease, but refers to more than 100 diseases in which genes have mutated and certain cells grow out of control. These cells can spread throughout your body. They can destroy normal body tissue and interfere with normal organ functioning.

Cancer and tumours are often linked. A mass of cancerous cells is commonly called a tumour. In some cases, tumours start in one part of the body and then spread to other locations, a process called metastasis. Not all tumours are cancerous and some do not spread. Not all cancers produce tumours. For example, leukemia is a cancer of the blood cells.

There are many possible causes of cancer. Some you may be able to change, such as stopping smoking. Others cannot be changed, such as your family history, gender or age. No one can predict who will get cancer. But some people may be at a higher risk than others. If you are in a "high-risk" group you may want to talk with your doctor about things you can do to reduce your risk.

There are four main types of cancer:

• Carcinomas are the most common type of cancer. They may start in the skin or tissues that line or cover internal organs, such as the bowels, cervix, lungs, kidneys, breasts, ovaries or prostate gland.
• Leukemias are cancers that start in blood-forming tissue such as the bone marrow.
• Lymphomas and multiple myeloma are cancers that begin in the cells of the immune system. Lymphomas are cancers of the lymph system, which includes the spleen, lymph nodes, and lymph vessels. Multiple myeloma is a cancer of the plasma cells.
• Sarcomas are cancers that start in muscles, fat, blood vessels, bones, cartilage and other connective or supporting tissues. Cartilage is at the ends of your bones and grows to form new bone.

Cancer may be named for the part of the body where cancer started, such as breast cancer, lung cancer or prostate cancer. It may also be referred to by the type of tissue where the cancer started such as bone cancer or blood cancer.

Visit our list of specific cancers and cancer organization websites for more information.