What causes cancer?

You get cancer when normal body cells go wrong in some way, and turn into cancer cells.

We don’t fully understand how or why this happens, but there are several factors that make it more likely that cells will become cancerous.

These factors include: 

Your lifestyle

You are more likely to get cancer if, for example:

  • you use tobacco
  • you spend too much time in the sun, and don’t protect your skin
  • you are exposed to certain substances or chemicals – asbestos, for instance
  • you eat a diet high in fat and red meat, and low in fruit and vegetables.

Doing these things doesn’t mean you are certain to get cancer, but you will increase your risk.

Your age

Although people of any age can get cancer, it mainly affects older people. The longer you live, the more time your normal cells have to go wrong and become cancerous.

Certain viruses

Some viruses can help cells to become cancer cells. If you have hepatitis B, for example, you are more likely to get primary liver cancer. If you have the genital warts virus you are more likely to get cancer of the cervix. 

Your immune system

If you have a problem with your immune system you are more likely to get certain cancers. AIDS patients, for instance, have an increased cancer risk.

Your genetic make-up 

You may have inherited a tendency to get cancer. In other words, you may have been born with some of your body cells already damaged.

Having these damaged cells doesn’t mean you will definitely get cancer, but it makes it more likely.

Some of the cancers that may run in families are:

  • breast cancer

  • bowel (colorectal) cancer

  • ovarian cancer

  • some skin cancers

  • prostate cancer

  • retinoblastoma (a rare eye cancer).

Some people inherit illnesses from their families – and having those illnesses makes cancer more likely. For example, if you inherit chronic pancreatitis you have an increased risk of getting cancer of the pancreas.

For most people who get cancer, there is no family link. But there are regional genetics services throughout the UK where people with a significant family history of cancer may be offered screening. The aim of screening is to work out a person’s cancer risk and to discuss what might be done either to prevent cancer developing or to detect it early.

It isn’t possible – or useful – to screen everyone. What counts as ‘significant family history’ varies from cancer to cancer. For example, a woman with two first degree relatives who have had ovarian cancer may be suitable for screening. (A woman's first degree relative is a mother, sister or daughter). If you would like to read more about cancer and genetic testing, go to tests for cancers that might have a genetic cause.

Research is going on all the time into cancer and its causes. The more we understand, the better able we will be to treat it and prevent it in the future.

Inherited genetic risks of breast and ovarian cancer 

Macmillan has created an online tool called OPERA to help you assess your inherited genetic risk of breast and ovarian cancer, which is available on the Macmillan website. The software program offers you personalised information and support in the comfort of your own home. 

Note: If you are seriously worried about developing breast or ovarian cancer, you might want to talk to your GP. They will confirm your family history and might refer you to see a specialist.