'Cancer' is characterised by an overgrowth of cells that serves no useful purpose and is not under the normal control of cell growth and specialisation   .
Cancer is not a single disease, but a large number of diseases. Many types of cancer can spread throughout a person’s body.
Cancers are sometimes called 'tumours' and 'neoplasms', but these terms can also be used for non-cancerous growths. Neoplasms can be 'benign' (not cancerous) or 'malignant' (cancerous).
Generally, a cancer:
For all cancers, there are changes (mutation) of particular genes within a cell (there are four main types of genes that control cell growth)  .
This alteration damages the mechanisms that regulate normal cell growth and differentiation.
Mutations that make a person more likely to develop cancer may be inherited  .
They can also occur by themselves or after exposure to something in the environment (such as chemicals - as in tobacco smoke - radiation or some viruses).
As well, changes to a person’s immune system may reduce the body's ability to recognise and destroy abnormally mutated cells. (This can happen, for example, when a person has HIV/AIDS.)
After development of a primary cancer (such as cancer of the lung or breast), it can spread in three main ways:
Different kinds of cancer have different risk factors - that is, factors that are linked with their development - but a number of cancers share common risk factors   . (It is important to know that some people with one or more risk factors may never develop a cancer, and that other people who do develop cancer have no apparent risk factors. Even when a person who has a risk factor is diagnosed with cancer, there is no way to prove that the risk factor actually caused the cancer.)
Environmental factors are believed to be associated with around three-quarters of all cancers . These include:
Some cancers may be a result of inherited genetic faults, which can also play a part in the development of cancers more linked with environmental factors. The causes are unknown for many cancers.
Important risk factors for cancer are tobacco use (through inhaled chemicals), unhealthy diet, and lack of physical activity . It is believed that about one-third of all cancer deaths are related to dietary factors and lack of physical activity in adulthood.
Most cancers are diagnosed when a person experiences symptoms associated with:
Clinical information is important to the diagnosis of cancer, which is usually confirmed by special tests (for example, a chest x-ray for lung cancer or examination of cells from a cancerous lump in the breast)  .
Even before a person has symptoms from a cancer, it may be detected by what are called ‘screening tests’  . Examples are Pap smears for cervical cancer or colonoscopy for bowel cancer. (Colonoscopy involves the insertion by a doctor of a long, flexible, lighted tube into a person’s back passage to look at the inside of their bowel.)