Cancer is caused by both intrinsic and extrinsic factors. Intrinsic factors include genetic make-up, diet and hormones while extrinsic factors include exposure to chemicals (like smoking, asbestos), radiation, and infection (e.g. viruses or bacteria).

- Genetic make-up:
A cell becomes cancerous only if it has a number of genetic mutations. Sometimes one is born with one of these mutations which increases your risk for that type of cancer. This means your chance of getting breast cancer increases if someone in your family is affected by it.
This is true of other common cancers like colon cancer which affects people with a particular type of genetic predisposition.

- Diet:
Experts say that change in diet could prevent one in three cancer deaths in the UK. We consume too many animal fats and ignore fresh fruit and vegetables which increases our risk of cancer.
Fruits and vegetables, foods containing fiber and vitamins A, C, E and D should be consumed more as they protect us against the disease. We should avoid eating foods with fats (especially animal fats), red and processed meats and alcohol because they raise the risk factor.
Your chance of getting stomach and oesophagus (gullet) cancer if you consume pickled food as they tend to be saltier.

- Immune system:
People with weak immune systems are more likely to get some forms of cancer. People who have gone through organ transplants and take drugs to suppress their immune systems to stop organ rejection too are at increased risk to get cancer. People who are born with rare medical syndrome like AIDS which affects their immunity are also at the risk of getting cancer.

- Carcinogens:
A 'carcinogen' is something that can help to cause cancer. One of the powerful carcinogens is tobacco smoke and so is asbestos - which means that smokers and those working in asbestos factories are at the risk of getting cancer. Though not all smokers and workers in asbestos factories get cancer as there are other factors too at work.

- Age:
Most types of cancer become more common as we get older because the changes that causes a cell to become cancerous take a long time to develop.

- Viruses:
Viruses too can help cause some cancers. Virus can cause genetic changes in cells that make them more likely to become cancerous. Some cancers and viruses are linked like cervical cancer and the genital wart virus, primary liver cancer and the Hepatitis B virus,T cell leukemia in adults and the Human T cell leukemia virus.
Many people can be infected with a cancer-causing virus but never get cancer as the virus only causes cancer in certain situations.
The American Cancer Society, in a recent report, said at least half of all cancer deaths in the US could be avoided if people don't smoke, undergo cancer screening tests, reduce levels of obesity and overweight, improve nutrition and increase their physical activity.