Quit smoking, drink less alcohol and maintain a healthy diet.
For years, this has been the mantra to reduce our risk of cancer, a disease that now affects one in two of us at some point in our lives.
But while that’s good advice based on a lot 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 worldwide (and about 1.4 million new cases of the disease worldwide each year) are due to an infection caused by a virus, bacteria or some other organism that weakens the body’s defenses enough that cancer cells sneak in and begin multiplying.
This doesn’t mean that cancer itself is contagious (or that you can catch it from other people), but it does mean that the disease can be triggered in part by certain microbes we encounter in our everyday lives.
The germ that could be causing a problem is a particular strain of E. coli, best known for causing outbreaks of food poisoning from undercooked meat and contaminated vegetables and salads.
Now research suggests this could explain why cases of bowel cancer appear to be increasing among young people.
Bowel cancer kills almost 17,000 people a year in the UK and, although it is usually associated with older people (NHS screening starts at age 54), cases in people aged 25 to 49 have risen by 22 per cent since the early 1990s.
And according to Dr Charles Swanton, clinical director at Cancer Research UK, one of the reasons could be a bacteria that some of us pick up in childhood.
The germ in question is a particular strain of E. coli, best known for causing outbreaks of food poisoning caused by undercooked meat and contaminated vegetables and salads.
The causative strain, called PKS-positive E. coli, is unclear how it is transmitted or develops (though some studies have linked it to Western-style diets; the theory is that diets high in red or processed meats, sugar, and refined grains, but low in vegetables, beans, or legumes, create an inflammatory environment in the gut that can allow harmful bacteria like PKS-positive E. coli to thrive).
And laboratory research suggests the bacteria release a toxin that damages cells lining the gut, increasing the likelihood that they will become cancerous over time.
A 2022 study by scientists at Harvard Medical School in Boston, USA, published in the journal Gastroenterology, found that the risk of developing bowel cancer increased after people were infected with this type of E. coli.
Researchers followed nearly 135,000 volunteers over a four-year period and compared bowel cancer rates to dietary habits. The results showed that those who ate a high-fat, high-sugar diet were not only more likely to develop bowel cancer, but were also almost 3.5 times more likely to have traces of PKS-positive E. coli DNA in their tumours.
Dr Charles Swanton, chief medical officer at Cancer Research UK, said: “There is emerging data showing that PKS-positive E. coli can induce mutations in intestinal cells which in turn could 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 emerging data showing that PKS-positive E. coli can induce mutations in intestinal cells which in turn could 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 (affecting around 3,300 women a year in the UK) is due to a handful of the approximately 150 types of human papillomavirus (HPV), an infection that is transmitted by close contact, often during sexual intercourse.
Smoking also increases risk, but cervical cancer typically forms when HPV enters healthy cells and hijacks their molecular machinery to produce even more virus particles, disrupting normal cell functioning in the process.
Other strains of HPV are largely responsible for most cases of cancer affecting the penis, vagina, anus, and throat.
Fortunately, an HPV vaccine for girls aged 12 to 13, which covers the 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 has also been offered to boys of the same age since 2019.
Scientists say it works so well that cervical cancer (which killed Big Brother star Jade Goody aged 27 in 2009) could even be eradicated in the UK in the next few years.
“Typically it’s viruses, like HPV, that persist for long periods in our bodies that 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, in which the virus remains inside the cells for years. Cancer does not develop from a cold.”
Hepatitis C, a chronic viral infection affecting 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’s critical to identify hepatitis C infections (symptoms include fever, abdominal pain, fatigue and loss of appetite) as early as possible. Treatment with antiviral pills 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 (leading to a sore throat and fatigue that can last for months)) is known to play a role 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 against Epstein-Barr infection, although several experimental vaccines are in clinical trials.
While viruses are responsible for many more cases of cancer than other organisms, some bacteria can also increase the risk of developing cancer.
An estimated one in 20 cases of stomach cancer, a disease that affects more than 6,000 people in the UK each year, is caused by long-term exposure to a gut bacteria called Helicobacter pylori, which can be passed from person to person through saliva and mouth-to-mouth contact.
It is believed that prolonged inflammation in the stomach, caused by the microorganism, predisposes the cells of the stomach lining to become malignant.
However, H. pylori infection is easily treated with a two-week course of antibiotics. The problem is that many people don’t know they are infected because it produces few symptoms.
So how concerned should we be about the risk of contracting cancer-causing organisms from other people or from the foods we eat?
Professor Young says simply being infected with a virus or bacteria does not automatically mean you are likely to develop cancer.
Infection, he says, is only a 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 as a result of it,” 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 HPV vaccine, you can stop cancer from forming.”
Stephen Griffin, professor of cancer virology at the University of Leeds, added: “Long-lasting viruses, such as HPV or hepatitis C, increase the hallmarks of cancer, such as DNA not being repaired correctly, cells multiplying when they shouldn’t, and inflammation.”
But while some viruses cause cancer, others are used to kill it.
For decades, scientists have been working on developing so-called “oncolytic viruses,” taking common viruses (like herpes simplex, which causes cold sores) and turning them into anti-tumor weapons.
First, the virus is adapted – or weakened – in the laboratory so that it does not cause a widespread infection in the body.
It is then injected directly into the tumor. The virus penetrates the cancer cells and grows to the point where the cell soon dies. At the same time, it draws the immune system’s attention to the presence of the cancer (tumor cells usually hide from the body’s defenses) so that it can send fighter cells to attack and destroy the cancer.
The first virus-based cancer cure was launched in the UK in 2015. Known as T-VEC (or talimogene laherparepvec), it is a treatment for malignant melanoma skin cancer when it has spread to other parts of the body.
A 2023 study showed that 22 percent of patients who received the T-VEC injection and surgery to remove advanced melanoma were still alive after five years, compared with just 15 percent of those who had surgery alone, the journal JAMA Oncology reported.
Other diseases being treated with viruses include liver cancer and brain cancer. “There are a lot of promising trials going on using viruses to treat cancer,” says Professor Griffin. “They are injected into the tumour and seem to have a very good response rate.”