What Causes Cancer?

This leaflet gives a brief overview of the causes of cancer. There are separate leaflets on the different types of cancer.

Cancer is a disease of the cells in the body. The body is made up from millions of tiny cells. Different parts of the body such as organs, bones, muscles, skin, and blood are made up from different specialised cells. All cells have a centre called a nucleus which contains genes made from DNA. The genes control the functions of the cell.

There are many different types of cells in the body, and many different types of cancer which arise from different types of cell. What all types of cancer have in common is that the cancer cells are abnormal and multiply out of control.

Each cancer is thought to first start from one abnormal cell. What seems to happen is that certain vital genes which control how cells divide and multiply are damaged or altered. This makes the cell abnormal. If the abnormal cell survives it may multiply out of control into a cancerous (malignant) tumour.

We all have a risk of developing cancer. Many cancers seem to develop for no apparent reason. However, certain risk factors are known to increase the chance that one or more of your cells will become abnormal and lead to cancer. Risk factors include the following:

Chemical carcinogens

A carcinogen is something (chemical, radiation, etc) which can damage a cell and make it more likely to turn into a cancerous cell. As a general rule, the more the exposure to a carcinogen, the greater the risk. Well-known examples include:

  • Tobacco. If you smoke, you are more likely to develop cancer of the lung, mouth, throat, oesophagus, bladder and pancreas. Smoking is thought to cause about 1 in 4 of all cancers. About 1 in 10 smokers die from lung cancer. The heavier you smoke, the greater the risk. If you stop smoking, your risk goes down considerably.
  • Workplace chemicals such as asbestos, benzene, formaldehyde, etc. If you have worked with these without protection you have an increased risk of developing certain cancers.


The older you become, the more likely that you will develop a cancer. This is probably due to an accumulation of damage to cells over time. Also, the body's defences and resistance against abnormal cells may become less good as you become older. For example, the ability to repair damaged cells, and the immune system which may destroy abnormal cells, may become less efficient with age. So, eventually one damaged cell may manage to survive and multiply out of control into a cancer. Most cancers develop in older people.

Lifestyle factors

Diet and other lifestyle factors (and, as mentioned, smoking) can increase or decrease the risk of developing cancer. For example:

Some examples of studies looking at lifestyle factors and cancer:

A large review of data (Parkin et al - cited below) found that about 4 in 10 cancers diagnosed in the UK each year - over 130,000 in total - are caused by avoidable lifestyle factors and choices including smoking, alcohol, weight and diet. Quote from lead author Professor Parkin ... "Many people believe cancer is down to fate or 'in the genes' and that it is the luck of the draw whether they get it. Looking at all the evidence, it's clear that around 40% of all cancers are caused by things we mostly have the power to change."

One large research study (Kirkegaard et al - cited below) followed up over 55,000 people for 10 years. It looked at lifestyle factors and rates of cancer. The study concluded that by following recommendations on keeping physically active, keeping weight in check, not smoking, drinking alcohol in moderation and having a healthy diet, the risk of developing bowel cancer could be reduced by as much as 23%. But, the study found that even improving in some of these lifestyle factors had some reduction in risk.

A study (Schütze et al - cited below) that followed up 363,988 people found that about 10 in 100 of all cancers in men and about 3 in 100 in women in western Europe are caused by people drinking alcohol. Most cancer cases were in people who drank higher than the recommended upper limits. But the study found that even drinking more than two units a day for men and more than one unit a day for women significantly increased the risk of developing certain cancers.

A study (Pan et al - cited below) that followed up over 100,000 people in the USA suggests that cutting the amount of red meat in most people's diets to 42 g per day (equal to about one large steak a week) could significantly reduce the incidence of certain cancers.


Radiation is a carcinogen. For example, exposure to radioactive materials and nuclear fallout can increase the risk of leukaemia and other cancers. Too much sun exposure and sunburn (radiation from UVA and UVB) increase your risk of developing skin cancer. The larger the dose of radiation, the greater the risk of developing cancer. But note: the risk from small doses, such as from a single X-ray test, is very small.


Some germs (viruses and bacteria) are linked to certain cancers. For example, people with persistent infection with the hepatitis B virus or the hepatitis C virus have an increased risk of developing cancer of the liver. Another example is the link between the human papillomavirus (HPV) and cervical cancer. Most (possibly all) women who develop cervical cancer have been infected with a strain (subtype) of HPV at some point in their life. Another example is that a germ (bacterium) called Helicobacter pylori is linked to stomach cancer.

One research study estimated that about one in six cancers - two million a year globally - are caused by largely treatable or preventable infections. They estimated that four infections - HPV, H. pylori, and hepatitis B and C viruses - accounted for 1.9 million cases of cervical, stomach and liver cancers in 2008. Most of these were in the developing world. Initiatives such as immunisation against HPV and hepatitis B are helping to combat these infections.

But, most viruses and viral infections are not linked to cancer.

Immune system

People with a poor immune system have an increased risk of developing certain cancers. For example, people with AIDS, or people on immunosuppressive therapy.

Your genetic make-up

Some cancers have a strong genetic link. For example, in certain childhood cancers the abnormal gene or genes which may trigger a cell to become abnormal and cancerous are inherited. Other types of cancer may have some genetic factor which is less clear-cut. It may be that in some people their genetic make-up means that they are less resistant to the effect of carcinogens or other factors such as diet.

Not everybody who has contact with a potential cancer-causing substance (carcinogen) or has an unhealthy lifestyle will develop cancer. For example, not all smokers develop cancer of the lung. In fact, we are all probably exposed to low doses of carcinogens a lot of the time.

The body has certain mechanisms which may protect us from developing cancer. For example, it is thought that many cells which are damaged by carcinogens can repair themselves. Also, the body's immune system may be able to destroy some types of abnormal cells before they multiply into a tumour. Perhaps one carcinogen may only damage one gene, and two or more genes may need to be damaged or altered to trigger the cells to multiply out of control.

In many cases it is likely that there is a combination of factors (such as genetic makeup, exposure to a carcinogen, age, diet, the state of your immune system, etc). These may play a part in triggering a cell to become abnormal, and allowing it to multiply out of control into a cancer.

Now read about Malignancy of Unknown Origin

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Original Author:
Dr Tim Kenny
Current Version:
Dr Roger Henderson
Peer Reviewer:
Dr John Cox
Document ID:
4806 (v43)
Last Checked:
20 October 2014
Next Review:
19 October 2017
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Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. Patient Platform Limited has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.