Cancer is a term for diseases in which abnormal cells grow and divide without control. This is usually the result of damage to a number of regulatory mechanisms within the cell . These damaged cells grow to form a tumour - an abnormal mass of tissue which can be cancerous or benign (non-cancerous). Unlike the cells in benign tumours, cancer cells can invade nearby tissues and spread via a process known as metastasis. Metastasis occurs when cells become detached from the initial tumour and are carried through the bloodstream and lymphatic system to other parts of the body . This eventually interferes with the function of normal cells and can lead to the death of the patient .
There are over 100 different types of cancer, each with its own pattern of growth and spread . While the risk factors for different cancers may be shared or unique, the cause may still be unknown  . The risk factors for cancer that are known can be broadly divided into environmental and internal (host) factors. Environmental factors include chemicals, radiation and viruses . Chemicals that have been identified as carcinogenic (cancerous) include tobacco smoke, alcohol (if consumed excessively) and asbestos, as well as certain industrial chemicals and medical drugs. Diet is also thought to initiate or promote various cancers  . Internal or host factors include hormones, immune conditions and inherited mutations . While some cancers, such as breast and colon cancer, are thought to have a strong 'familial predisposition', there is still no evidence that cancer is normally 'programmed' in the cells .
The frequency of cancer increases with age, with relatively few people acquiring cancer before the age of 30 years. This is partly because it can take many years to acquire the multiple abnormalities that generate cancer cells . Furthermore, the probability of being exposed to the risk factors for cancer also increases with time .
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