What Causes Cancer ?

A Simple Explanation

There is not a single answer to the question “what causes cancer?” In total, it is estimated that there are 200 different types of cancer. Cancer can arise in any cell or tissue of the body. Something that affects one tissue may not affect another.

For example, breathing in tobacco smoke may contribute to causing lung cancer. Overexposing your skin to the sun could cause a melanoma on your back. However, smoking won’t cause melanoma, and the sun won’t give you lung cancer.

Most illnesses that affect us as humans are what we call “multifactorial.” This means that there are many factors involved in causing the disease. The development of cancer is no exception, and this means that there is no single cause for any one type of cancer.

Knowing what causes cancer can in some cases help lower the risk of developing the disease. This article contains information about the various factors that can cause the development of cancer, and which factors you can avoid.

First of all, we need to define the term “carcinogen.” A carcinogen is something (a chemical for example) that can contribute to what causes cancer. Tobacco smoke is a powerful carcinogen, however not everyone that smokes gets lung cancer. Therefore there must be other factors at play. Some of these are described below:


The underlying cause of cancer is genetic mistakes in our cells that cause them to grow uncontrollably. The longer we live, the more time there is for genetic mistakes to accumulate in our cells. It is a fact that most types of cancer become more common as we get older.

The reason for this is that the changes that make a cell become cancerous take a long time to develop, and there must be a number of changes to the genes within a cell before it can grow uncontrollably. These mistakes, or mutations, can happen by accident when the cell is dividing, or when the cell has been damaged by carcinogens and the damage is passed on to future daughter cells.

The day-to-day environment

There are factors in our environment that we are exposed to every day that may contribute to what causes cancer. These include:

  • Ultraviolet radiation from the sun
  • Tobacco smoke
  • Workplace chemical hazards
  • Asbestos
  • Natural and man-made radiation

Obviously some of these are avoidable and others aren’t. It is a good idea to avoid these as much as you can.

Genetic make-up

As I mentioned above, there need to be a number of genetic mutations in a cell in order for it to become cancerous. Sometimes these mutations are inherited from an individuals parents which gives them a statistically higher chance of developing a particular cancer, depending on which gene has a mutation in it. Technically, it is called genetic predisposition.

Mutations in the Breast and Ovarian Cancer susceptibility genes 1 and 2 (BRCA1 and BRCA2) are an example of genes that give women a higher risk of developing breast and ovarian cancer than women without mutations. However, most women with breast or ovarian cancer do not have a mutated BRCA1 or BRCA2 gene. Only about 5-10% of breast and ovarian cancers are due to these mutations.

There are also a number of other cancers where some people have a genetic predisposition including colon and skin cancer.

The Immune System

Problems with an individual’s immune system is another factor for what causes cancer. There are a number of conditions which will increase the risk of developing cancer including people who:

  • Have had an organ transplant and take drugs to suppress their immune system to prevent organ rejection
  • Have AIDS or HIV
  • Are born with rare medical syndromes which affect their immunity

One important thing to know is that our immune system plays a crucial role in preventing cancer by getting rid of cells that are abnormal and possibly becoming cancerous. When the immune system isn’t working normally, the chances of a cancer cell not being recognised and eliminated by the immune system increases


Viruses are involved in the development of some cancers. What causes cancer in these cases is the genetic changes that the virus causes in an individual’s cells.

Some viruses and the cancers they can cause are:

  • Human Papilloma Virus (HPV) linked to cervical cancer and other genital and anal cancers
  • Hepatitis B and C viruses and primary liver cancer
  • Epstein-Barr virus and lymphomas
  • Human T-cell leukaemia virus and T-cell leukaemia

Only in certain situations are these viruses what causes cancer. Not everyone that is infected with these viruses will get cancer. Having safe practices to prevent being infected with these viruses can help prevent these cancers.

Bacterial Infection

In the past, bacterial infections were not thought to be a factor in what causes cancer. However, studies have now shown that a stomach infection with Helicobacter Pylori leads to inflammation of the stmach lining, which increases the risk of stomach cancer.

Scientists are also looking at the possibility of some bacteria producing cancer causing substances in some people. This research is still in its early days.

Bacterial infections can be cured by antibiotics, so getting rid of an infection could be a way of reducing the risk of cancer.

Body weight, diet and physical activity

These days in western society, many of us eat too much red meat and processed foods, and not enough fruit and vegetables. A diet like this is known to be a contributing factor to what causes cancer. Alcohol can also increase the risk of developing certain types of cancer (e.g. liver, breast and bowel). Maintaining a healthy body weight, having a healthy diet and undertaking regular exercise is estimated to prevent 1 in 3 cancer deaths in the UK.

Being overweight or obese can increase the risk of bowel, pancreatic, kidney, gallbladder, breast and uterine cancers amongst others.