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Cancer, in essence, is the human body going wrong. It is not possible to 'catch' cancer (although in certain cases a predisposition to developing it may be inherited) It results from the regulatory and control systems of your body failing to function as they should. As such, while certain factors may increase the risk, absolutely anyone has the potential to develop cancer.

In general terms there are two types of tumour (cancerous growth) which may be defined as either benign or malignant. Benign tumours are far less threatening, and are rarely fatal. This is for a number of reasons. Firstly, cells in a benign tumour are more ordered, and resemble the tissue from which it came. Also, while the growth rate of its cells is above normal, it is not as rapid as malignant tumours. One of the typical characteristics of a benign tumour is that it is often (either wholly or partially) surrounded by capsule formed of fibroblasts, which keep it in place to a limited extent. The vital characteristic of a benign tumour however is that it does not metastasize, (more later) and as such is much easier to treat. Malignant tumours, by contrast, are highly disorganised, have a very rapid rate of growth, have no capsule, and are likely to metastasize.
Tumours are most likely to develop in epithelial cells, which form the skin, and line the lungs, ovaries, gut, etc. This is because these types of tissue have a naturally higher rate of growth, as they are continuously being replaced, and so are more likely to mutate (see below).

Cancer is the result of uncontrolled cell growth. A normal cell contains factors which tell it when to divide. There are also factors which inhibit this division, which prevents excessive cell growth. These factors are controlled by a number of genes. When a cell divides it is possible that it fails to accurately replicate its genetic material. When this happens, the cell is said to have mutated. In most cases, the cell cannot survive with this mutation and simply dies. However, if a cell develops a mutation in one of these specific 'control' genes, this may lead to uncontrolled growth. For example, the 'divide' instruction may be permanently 'switched on', or the inhibitor may be permanently 'switched off'. Either case will cause the cell to continue dividing at a much faster rate than that of the cells around it.
Another control factor present in normal cells but absent from cancer cells is contact inhibition. Simply, this stops cells from dividing when they're in contact with other cells around it, thus also maintaining a controlled rate of growth. However, as cancer cells lack this, they continue to grow rapidly, pushing outwards and invading the surrounding tissues. If this continues, this will cause great damage to the tissues, resulting in the death of the affected normal cells. The extent of the damage will depend on the tissues in which it occurs. For example, if this takes place in the liver, the cancer cells will spread into the liver, killing the normal cells, and eventually taking up so much space that there aren't enough liver cells to allow it to function, resulting in organ failure and the death of the patient.

A characteristic present in cancer cells is that of angiogenesis, which is the process of creating new blood capillaries. This takes place in normal cells during wound healing, and normal growth. However, cancer cells have the ability to constantly create a new blood supply. This is vital for the cancer's survival, as the blood supply brings the cells the energy and nutrients which they need to keep dividing at their accelerated rate. This can also lead to one of the more devastating aspects of cancer: Metastasis.
Metastasis is where a few cancer cells manage to permeate the capillary wall. Once inside the capillary the cell(s) will travel through the blood system until they become lodged at another site in the body. They then continue to grow as before, meaning that the patient now has cancer in more than one location. At this point treatment becomes much more difficult, and the prognosis (the likelihood of survival) becomes far less favourable.

This process may not only be caused by mutations in the cell however. Changes in the growth of cells may also occur due to chronic irritation to those cells. Normal tissues have the ability to alter their growth rate in response to external factors. For example, if you cut yourself, the growth rate of the skin cells in that area will increase so that the wound will heal quickly. Once the healing process is complete, the growth rate returns to normal. If the skin is exposed to persistent irritation - such as the rubbing of a shoe - the cells will adapt by increasing their growth rate and producing a thicker, harder area of skin, in order to protect itself from the irritation. This process is called hyperplasia, and is reversible, so that if you stop wearing the shoe, the skin returns to normal. However, if the irritation is more extreme, such as smoke regularly entering the lungs, a process known as metaplasia occurs. This is where cells actually convert to a different type as a response to the irritation. This happens in the lining of the lungs in smokers. Due to their new form, the cells are no longer able to clear away debris so effectively, and so smokers are more likely to suffer from illnesses such as bronchitis. However, this process is also reversible, and if a smoker quits, given enough time, the lung lining will return to normal. An even more extreme development of this is dysplasia, which occurs after prolonged irritation, such as in chronic or long-term smokers, or miners who are exposed to large quantities of dust on a very regular basis. Not only do these cells change their form, but they also show a disordered growth pattern, and become irregular in shape, with large, dark nuclei. These cells may also show an increased growth rate. As dysplasia can appear similar to cancerous tissue, it is also known as precancerous. This state of dysplasia is irreversible and it is very easy for this tissue to become cancerous. Once this has occured, and a tumour begins to form, the cell change is known as neoplasia.