We do know that there are a number of 'risk factors' that can make the faults more likely. The main one of these is being older: two-thirds of all cancers are diagnosed in people over 65 years of age because, like any machine, our systems become less reliable over time.

It is a common myth that injuries can cause cancer. Cancer is not caused by a fall, fracture, bruise or bump. Sometimes it is when a person seeks medical help for an injury that a tumour is discovered, but it may have been there for a long time and is not due to the injury.

Some people believe that cancer may be caused by stress, but there is not yet any reliable evidence to support this.

The other risk factors can be organised into the following categories:

• environmental
• diet and lifestyle
• genes and hereditary cancer
• viruses

Environmental risk factors

Coming into contact with specific substances or environments, particularly over time, can trigger the faults in our cells that lead to cancer.

For example, we know that most skin cancers are caused because of prolonged exposure to the sun. In this case it is the ultraviolet radiation in the sunlight that can trigger the faults that lead to cancer. Trigger substances or agents are known as carcinogens.

There are hundreds of substances that are suspected of being carcinogenic, usually at high doses, but it can be hard to establish clear evidence about whether or not they directly cause cancer, so it pays to be wary of sensational headlines. However in some cases the facts are clear, for example tobacco contains over 40 carcinogens.

Some people may have come into contact with carcinogens through their work. We know that certain chemicals can cause specific cancers. For example one of the main causes of bladder cancer is exposure to chemicals that were used in dye factories, rubber, gasworks, plastics and other chemical industries. These chemicals were banned in the UK in 1967.

However, it can take about 25 years after exposure to these chemicals for bladder cancers to develop. People who were exposed to certain chemicals through work and then develop bladder cancer may be eligible to claim industrial disease benefit from the Department for Work and Pensions.

Exposure to high concentrations of radon, a naturally occurring radioactive gas, increases the risk of lung cancer. Radon levels vary considerably between different parts of the country. The Department for Environment, Food and Rural Affairs (Defra) can provide you with information on radon levels in your area. Visit the Defra website to find out more.

Diet and lifestyle

Every day we make decisions about what we are going to do and eat. Research shows that some of these decisions can increase or reduce our risk of developing cancer.

Smoking - Everybody knows that smoking is bad for you. The good news is that even at 60, giving up smoking can reduce the risk of lung cancer.

Tobacco can trigger changes to cells. When a series of these changes occurs, it triggers lung cancer. But research shows that people who have not already developed the condition are unlikely to do so after they quit.

Smoking causes a third of all cancers. As well as lung cancer it is the major cause of cancers of the mouth, nasal passages, larynx, bladder and pancreas. It also plays a part in causing cancers of the oesophagus, stomach, kidney and in leukaemia.

Half of all smokers die as a result of their tobacco use, so it really is worth quitting while you're ahead.

For help on giving up go to the NHS website Giving up Smoking or call the NHS Smoking Helpline free on 0800 169 0 169.

Diet - After smoking, what people eat is the next biggest contributor to cancer deaths, and may be responsible for up to a third of all cancer deaths in the UK.

Researchers are working very hard to find out the effect that specific foods have on our risk of cancer. At the moment it is clear that eating plenty of fruit and vegetables and cereals, and reducing fat, salt and sugar is the best way to keep your risk of cancer low.

Eating lots of red or processed meat is a risk factor for bowel cancer. There is also some evidence that breast, lung, prostate and pancreatic cancers are linked to a diet high in red or processed meat.

Onions and garlic may help to protect against some cancers of the stomach and bowel, although more studies need to be done into this. There is good evidence that dietary fibre, particularly from vegetables, can help to protect against cancer of the colon.

The substances that give fruit and vegetables their different colours may help protect against cancer. They are called carotenoids and to make sure you get the benefit from them it is a good idea to eat fruit and vegetables of lots of different colours. Fruit and vegetables also contain antioxidants, which are thought to help protect our cells from developing the faults that could potentially cause cancer.

Careful studies have failed to find any definite link between coffee drinking (even when the consumption has been heavy) and any type of cancer.

It is never too late to begin healthy eating - visit our healthy eating section to find out more.

For more information on vitamins and mineral and whether or not to take supplements have a look at our section on nutrients.

Sunlight - Skin cancer is now the most common form of cancer in the UK. The number of cases here has doubled in just 20 years. Many of these cases are cured, but, in the last five years, 8,000 people have died because of skin cancer.

Skin cancer appears many years after the skin receives too much ultraviolet radiation, which is usually from sunlight or sun beds. If skin cancer is caught early, it can almost always be successfully treated.

Every time you are sunburnt you could seriously increase your chances of skin cancer. So it clearly makes sense to limit your exposure to harmful UV rays. The best idea is to follow some basic sun safety rules:

• Firstly – never burn, sunburn means your skin has been damaged by UV rays so don't wait till your skin feels uncomfortable to cover up.
• Wear a hat to protect your head, ears and neck.
• Cover up with cool loose fitting clothes – this is the best way to stay cool and avoid heatstroke.
• Use a sunscreen with an SPF of 15 or more – put it on before you go out, renew it regularly and use generous amounts.
• Protect your eyes by wearing sunglasses – exposure to the sun can damage your retina and cause cataracts.

If you've got a tan, you've already damaged your skin and a tan doesn't protect you against further damage, so start following the sun safety rules.

As an added bonus, limiting your exposure to the sun will keep your skin looking young!

Exercise - We should probably all be doing far more exercise than we are. Being more active more often will help you to be generally fitter and to reach or keep to a healthy body weight. This can help to prevent the risk of specific cancers such as breast cancer for women who have been through the menopause and also of cancer of the uterus. It may also help protect against cancer of the bowel and kidney.

Research indicates that having a healthy diet, exercising regularly and being a healthy weight will help to lower your risk of developing a range of different cancers. Of course it will also help to prevent you from developing other health problems such as heart disease and diabetes.

For more information on exercise, see our exercise section.

Alcohol - Drinking alcohol is linked with an increased risk of some, but not all cancers. Regularly drinking over the recommended limits for several years increases the risk of cancers of the mouth and throat and may also be linked to breast cancer.

People who are dependent on alcohol or drink large amounts regularly have an increased risk of liver and bowel cancer.

Genes and hereditary cancer

Some cancers are more likely to develop in certain families than others, which shows that inherited faulty or altered genes can also play a role in causing cancer.

Breast cancer, ovarian cancer and bowel cancer have all been shown to act in this way. This does not mean that they definitely will run in a family. Less than ten out of 100 cases of cancers are clearly linked to an inherited gene change. However, if you have two relatives with the same type of cancer on the same side of your family it is worth talking to your doctor about your risk of developing that cancer.

Viruses

Cancer is not infectious in any way. However some viruses are known to increase the risk of specific cancers, although this is relatively rare.

A virus known as the Epstein-Barr virus has been linked to some kinds of lymphoma, which is cancer of white blood cells in the lymph system.

The human papilloma viruses (HPVs) are a group of more than 80 different types of virus, including those that cause genital warts. They can be transmitted through sexual intercourse and have been linked to cervical cancer. It is estimated that up to 15 per cent of women aged 20 to 30 women and up to 6 per cent of women over 40 carry the virus. The majority do not go on to develop cancer.

The bacteria known as H-pylori, which can cause stomach ulcers, is also linked to a rare type of stomach cancer.