Revealed: The viruses that can cause cancer Experts now say one in five cases could be caused by a disease Our special report from leading doctors tells you how to protect yourself

Stop smoking, drink little alcohol and eat healthy.

This has been the mantra for years to reduce the risk of cancer, a disease that now affects one in two people at some point in their lives.

While that’s good advice, based on an abundance of solid evidence, it’s not always the case that cancer is the result of lifestyle, personal habits, or even genetic risks.

According to the American Society for Microbiology, nearly one in five cancer deaths worldwide (and about 1.4 million new cases of the disease occur worldwide each year) is caused by an infection caused by a virus, bacteria or other organism that weakens the body’s defenses enough to allow cancer cells to slip through and multiply.

This doesn’t mean that cancer itself is contagious – or that you can catch it from others – but it does mean that the disease can be caused in part by certain bacteria we pick up in our daily lives.

The bacteria that could be the culprit is a particular strain of E. coli – best known as the cause of food poisoning outbreaks via undercooked meat and contaminated vegetables and salads

Research now suggests that this could explain why colon cancer is becoming more common in young people.

Nearly 17,000 people die from bowel cancer each year in the UK. The disease is usually diagnosed in older people (NHS screening starts at age 54), but the number of cases in people aged 25 to 49 has risen by 22 per cent since the early 1990s.

According to Dr Charles Swanton, chief clinical scientist at Cancer Research UK, one reason could be a bacteria that some of us pick up in childhood.

The bacteria in question is a particular strain of E. coli, best known as the cause of food poisoning from undercooked meat and contaminated vegetables and salads.

The bacteria is called PKS-positive E. coli, and it’s not clear how this strain of bacteria is transmitted or develops (although some research has linked it to Western diets. The theory is that diets high in red or processed meat, sugar, and refined grains, but low in vegetables, beans, or legumes, create a pro-inflammatory environment in the gut that allows harmful bacteria like PKS-positive E. coli to thrive).

Laboratory studies show that the bacteria release a toxin that damages the cells in the intestinal wall, increasing the chance that they will eventually become cancerous.

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

Researchers followed nearly 135,000 volunteers over a four-year period and compared colon cancer rates with dietary habits. The results showed that people with a high-fat, high-sugar diet were not only more likely to develop colon cancer, but also nearly 3.5 times more likely to have traces of PKS-positive E. coli DNA in their tumors.

Dr Charles Swanton, head of clinical science at Cancer Research UK, said: ‘Emerging data shows that PKS-positive E. coli can cause mutations in gut cells that may in turn contribute to at least some of the cancer initiation processes.’

As Dr Swanton explained in a recent interview with the charity The Health Foundation: ‘There is data to show that PKS-positive E. coli can cause mutations in intestinal cells which in turn may contribute to at least some of the cancer initiation processes.’

But it is not the only form of cancer that can be caused by infections.

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

Smoking also increases the risk, but cervical cancer usually occurs when HPV invades healthy cells and hijacks their molecular machinery to produce more virus particles, disrupting the cells’ normal function.

Other strains of HPV are largely responsible for most cases of cancer of the penis, vagina, anus, and throat.

Fortunately, an HPV vaccine for girls aged 12 to 13, which covers the strains most commonly linked to cancer, has reduced the number of cases in England by around 90 per cent since it was introduced in 2008. Since 2019, it has also been offered to boys of the same age.

Scientists say it works so well that cervical cancer – which killed Big Brother star Jade Goody in 2009 at the age of 27 – could be eradicated in the UK in the coming years.

“Normally it’s viruses, like HPV, that stay in our bodies for long periods of time and are associated with cancer,” says Lawrence Young, a virologist and professor of molecular oncology at the University of Warwick.

“We’re talking about long-term viral infections where the virus stays in your cells for years. You don’t get cancer from a cold.”

Hepatitis C, a chronic viral infection that 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.

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

Meanwhile, the Epstein-Barr virus – the so-called “kissing bug” that causes glandular fever (which causes a sore throat and fatigue that can last for months) – is known to play a role in the development of around 40 percent of cases of Hodgkin’s lymphoma, an aggressive cancer that affects more than 2,000 people in the UK each year. There is currently no vaccine for Epstein-Barr infection, although several experimental jabs are in clinical trials.

Although viruses are much more likely to cause cancer than other organisms, some bacteria can also increase the risk of cancer.

It is estimated that one in 20 cases of stomach cancer, which affects more than 6,000 people in the UK each year, is caused by long-term exposure to a gut bacterium called Helicobacter pylori. This bacteria can be passed from person to person through saliva and mouth-to-mouth contact.

It is thought that long-term inflammation in the stomach, caused by the bacteria, makes the cells in the stomach lining more susceptible to malignancy.

Yet H. pylori is easily treated – with a two-week course of antibiotics. The problem is that many people don’t know they are infected, because it causes few symptoms.

How concerned should we be about the risk of contracting cancer-causing organisms from other people or from the food we eat?

Professor Young says that it is not the case that you get cancer if you become infected with a virus or bacteria.

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

“For example, about 95 percent of us have the Epstein-Barr virus in our bodies, but the vast majority of us will not develop cancer from it,” he says. “The virus is one link in a whole chain of events that causes cancer, but if you break that chain — like we’re doing with the vaccine for HPV — then you can stop the cancer from forming.”

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

Some viruses cause cancer, but others are used to kill cancer.

For decades, scientists have been working on developing so-called “oncolytic viruses.” They use common viruses (such as herpes simplex, which causes cold sores) to turn them into anti-tumor weapons.

First, the virus is modified – or weakened – in the laboratory so that it does not cause widespread infection in the body.

It is then injected directly into the tumor. The virus breaks into the cancer cells and grows to the point where the cell quickly dies. At the same time, it draws the immune system’s attention to the presence of cancer (tumor cells usually hide from the body’s defenses), so it can launch fighter cells to attack and destroy the cancer.

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

A 2023 study found that 22 percent of patients who received the T-VEC shot and also had surgery to remove advanced melanoma were alive after five years, compared with just 15 percent of those who had surgery alone, the journal JAMA Oncology reported.

Other diseases that are candidates for viral treatment include liver cancer and brain cancer. ‘There are many good studies going on with promising results using viruses to treat cancer,’ says Professor Griffin. ‘They are injected into the tumour and seem to have a very good response.’