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Cells grow then divide by mitosis only when we need new ones - when we're growing or need to replace old or damaged cells.

When a cell becomes cancerous, it begins to grow and divide uncontrollably. New cells are produced - even if the body does not need them.

A group of cancerous cells produces a growth called a tumour.

Cancer cells are undifferentiated - they do not carry out their normal function. Tumours can be either benign or malignant. Benign tumours grow slowly and do not spread. Malignant tumours grow faster and can spread to other parts of the body. This makes them more serious.

The diagram shows how cancer cells can invade surrounding tissue:

What causes cancer?

There are genetic factors that increase the likelihood of developing some cancers.

Chemicals and other agents that can cause cancer are called carcinogens.

Carcinogens cause cancer by damaging DNA. Carcinogens cause mutations to occur. A single mutation will not usually cause cancer - several are required. For this reason, we are more likely to develop cancer as we get older.

Something that increases the likelihood of developing a disease is called a risk factor. There are several risk factors for various types of cancers including:

Lifestyle factors

• Viruses linked with cancer, such as the human papilloma virus (HPV), being spread from person to person through sexual intercourse. HPV is linked to cervical cancer in women.
• The chemical carcinogens in cigarette smoke increasing the risk of lung cancer.
• Alcohol intake is linked with certain cancers.
• Exposure to ultraviolet radiation, part of which is ionising, during sunbathing or outdoor activities, leads to the development of skin cancers.
• Diet, including fat and salt intake, increases the risk of cancer.

Industrial and environmental factors at work

• Exposure to ionising radiation increases the risk factor.
• Exposure to chemical carcinogens, for example in asbestos dust.

There are also genetic risk factors for some cancers.