The viruses that can give you cancer - and how to protect yourself

Quit smoking, go easy on the alcohol and stick to a healthy diet.

For years, this has been the mantra for reducing our risk of cancer, a disease that now affects one in two of us at some point in our lives.

But while that's sound advice based on masses of solid evidence, it's not always the case that cancer is the result of lifestyle, personal habits or even genetic risk.

In fact, according to the American Society for Microbiology, nearly one in five cancer-related deaths globally (and around 1.4 million new cases of the disease around the world every year) are due to an infection caused by a virus, bacteria or some other organism that weakens the body's defences enough for cancerous cells to sneak past and start multiplying.

This doesn't mean cancer itself is infectious – or can be caught from others – but it does mean the disease can be partly triggered by certain bugs we pick up in our everyday lives.

Now research suggests this might explain why cases of bowel cancer appear to be on the increase in young people.

Bowel cancer kills almost 17,000 people a year in the UK and while normally associated with older people (NHS screening for it begins at the age of 54) cases in those aged between 25 and 49 have risen by 22 per cent since the early 1990s.

And according to Dr Charles Swanton, chief clinician at Cancer Research UK, one reason might be a bacterium that some of us contract in childhood.

The bug in question is a particular strain of E. coli – best known as a cause of food poisoning outbreaks via undercooked meat and contaminated vegetables and salads.

Called PKS-positive E. coli, it's not clear how the offending strain is transmitted or develops (though some studies have linked it with Western-style diets – the theory is that diets high in red or processed meats, sugar and refined grains, but low in vegetables, beans or pulses, create an inflammatory environment in the gut which may allow harmful bacteria like pks-positive E. coli to thrive).

And lab research suggests that the bacteria release a toxin that damages the cells lining the bowel, increasing the likelihood they will become cancerous over time.

A study in 2022 by scientists from Harvard Medical School in Boston, US, published in the journal Gastroenterology, found that the risk of developing bowel cancer increased after people had become infected with this type of E. coli.

Researchers tracked almost 135,000 volunteers over a four-year period, comparing bowel cancer rates with dietary habits. The results showed those on high-fat, high-sugar diets were not only more likely to get bowel cancer but almost 3.5 times more likely to have traces of DNA from PKS-positive E. coli in their tumours.

As Dr Swanton, speaking to the charity The Health Foundation, explained in a recent interview: 'There is data emerging that shows PKS-positive E. coli can induce mutations in gut cells that might, in turn, contribute to at least some of the cancer initiation processes.'

But it's not the only form of cancer that can be brought on by infections.

For example, almost every single case of cervical cancer – which affects around 3,300 women a year in the UK – is due to a handful of the 150 or so types of human papillomavirus (HPV), an infection spread by close contact, often during sex.

Smoking does also raise the risk, but cervical cancer usually forms when HPV gets into healthy cells and hijacks their molecular machinery to produce even more virus particles – in the process disrupting cells' normal function.

Other HPV strains are largely responsible for the majority of cases of cancers that involve the penis, vagina, anus and throat.

Thankfully, an HPV vaccine for girls aged 12 to 13, covering strains most associated with cancer, has reduced the number of cases in England by around 90 per cent since it was introduced in 2008. It's also been offered to boys of the same age since 2019.

Scientists say it works so well that cervical cancer – which led to the death of Big Brother star Jade Goody at the age of 27 in 2009 – could even be eradicated in the UK in the next few years.

'Normally it's viruses – like HPV – that persist for long periods in our bodies which are associated with cancer,' says Lawrence Young, a virologist and professor of molecular oncology at the University of Warwick.

'We are talking about long-term viral infections where the virus sits inside your cells for years at a time. You won't get cancer from having a cold.'

Hepatitis C – a chronic viral infection which affects more than 60,000 people in England – is another virus that causes long-term inflammation in the liver, leading to cirrhosis (or scarring) and, eventually in some cases, cancer cells.

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Doctors say it's crucial to identify hepatitis C infections (symptoms include fever, abdominal pains, fatigue and loss of appetite) as soon as possible. Treatment with antiviral tablets can clear the infection – and eradicate the cancer risk – in more than 90 per cent of people.

Meanwhile,the Epstein-Barr Virus – the so-called 'kissing bug' that causes glandular fever (triggering a sore throat and fatigue that can last months) – is known to play a part in the development of around 40 per cent of cases of Hodgkin lymphoma, an aggressive cancer that affects more than 2,000 people a year in the UK. There is currently no vaccine for Epstein-Barr infection although several experimental jabs are undergoing clinical trials.

While viruses account for many more cancer cases than other organisms, some bacteria can also raise your risk of developing cancer.

An estimated one in 20 cases of stomach cancer, a disease affecting more than 6,000 people in the UK annually, are brought on by long-term exposure to a gut bug called Helicobacter pylori, which can be passed from one person to another through saliva and mouth-to-mouth contact. 

It's thought prolonged inflammation in the stomach, triggered by the bug, predisposes stomach lining cells to become malignant.

Yet H. pylori is easily treatable – with a two-week course of antibiotics. The problem is many people have no idea they are infected as it produces few symptoms.

So how worried should we be about the risk of catching cancer-causing organisms from other people or the foods we eat?

Professor Young says simply becoming infected with a virus or bacteria does not automatically mean you're likely to get cancer.

The infection, he says, is just one small part of a much bigger process – involving exposure to numerous other risk factors or carcinogens – that leads to tumour formation.

'For example, around 95 per cent of us have Epstein-Barr virus in our bodies yet the vast majority of us will not develop cancer as a result,' he says. 'The virus is one link in a whole chain of events that cause cancer but if you break that chain – as we do with the vaccine for HPV – then you can stop the cancer forming.'

Stephen Griffin, professor of cancer virology at Leeds University, adds: 'Long-term viruses, like HPV or hepatitis C, increase the hallmarks of cancer – such as DNA not being repaired properly, cells multiplying when they shouldn't or inflammation.'

But while some viruses cause cancer, others are being used to kill it.

For decades, scientists have been working on developing so-called 'oncolytic viruses' – taking common viruses (such as herpes simplex, which causes cold sores) and turning them into anti-tumour weapons.

First the virus is adapted – or weakened – in the lab so it does not cause widespread infection in the body.

Then it is injected directly into the tumour. The virus breaks into the cancer cells and grows to the point where the cell soon dies. At the same time, it draws the attention of the immune system to the cancer's presence (tumour cells usually hide from the body's defences) so it can launch fighter cells to attack and destroy the cancer.

The first virus-based cancer remedy was launched in the UK in 2015. Known as T-VEC (or talimogene laherparepvec), it is a treatment for the skin cancer malignant melanoma when it has spread to other parts of the body.

A 2023 study showed 22 per cent of patients given the T-VEC jab as well as surgery to remove advanced melanoma were still alive after five years – compared with just 15 per cent in those who had surgery alone, reported the journal JAMA Oncology.

Other diseases being targeted for virus-treatment include liver cancer and brain cancer. 'There are lot of good trials coming through with promising results on using viruses to treat cancer,' says Professor Griffin. 'They are injected into the tumour and appear to have a very good response rate.'