What causes cancer?

Find out what causes cancer, how it forms in the body and why it can affect young people too.

  • Cancer starts in the body when cell production goes wrong
  • New cells are produced when they’re not needed
  • This can cause a mass of tissue called a tumour to develop

What causes cancer to start in the body?

Our bodies are made up of billions of cells. Brain cells. Nerve cells. Muscle cells. Bone cells. Gland cells. Reproductive cells. Everywhere you look in the human body, you’ll find cells – they’re our building blocks.

Normally, cells only grow and divide to make more cells when the body needs them. This keeps the body healthy, because damaged cells get replaced quickly.

Sometimes, though, things go wrong. New cells are produced when they’re not needed, and this can cause a mass of tissue called a tumour to develop.

What are tumours?

There are different types of tumour. Some are called benign or malignant. The benign ones aren’t cancerous. The malignant ones are. 

Benign tumours usually grow more slowly and are less likely to come back if they’ve been completely removed. They very rarely spread to other parts of the body, but if they haven’t been completely removed they can sometimes start growing again after treatment.

Malignant tumours usually grow faster. They are different to benign tumours, as they can sometimes spread to other parts of the body, and may come back after treatment.

And some cancers, like leukaemia, don’t form tumours at all, but affect the bone marrow (the spongy stuff inside your bones that produces blood cells) or the blood.

Why have I got cancer?

Cancer is a lot of things, but it’s not your fault.

Cancer itself is pretty common – around 1 in 2 people in the UK will get it at some point during their lives. But cancer in young people is much rarer, with less than 1% of all cancer cases each year affecting 13-24-year-olds.

We are learning more all the time, but often we don’t know exactly why one person gets cancer and another doesn’t.

Maybe you’re worried about things you’ve done, like smoking or drinking or taking drugs. These things do have links to cancer, but there’s almost no chance of them causing cancer in young people.

Spending too much time in the sun can put you at risk. So can having unprotected sex, as the HPV virus can be passed on, which can be linked to some forms of cancer.

There are some things that can increase the risk of getting cancer, known as ‘risk factors’. But even though it’s really unfair, cancer can just happen for no obvious reason.

This page was reviewed in 2024. It will reviewed again in 2027.

Patient Information Forum - Trusted Information Creator

PIF TICK accredited information

The Patient Information Forum is the UK membership organisation and network for people working in health information and support. The PIF TICK is the UK-wide Quality Mark for Health Information.

Find out more