A bacterium that can live quietly in the stomach for decades may have an unexpected connection with bowel cancer.

A new study estimates that exposure to Helicobacter pylori (H pylori) could be associated with more than one in five cases of bowel cancer worldwide.

That is a striking figure, particularly because H pylori is extremely common. But it comes with an important qualification: the study doesn’t prove H pylori causes bowel cancer. Instead, it estimates how much bowel cancer might be linked to the infection.

Bowel cancer, also called colorectal cancer, develops in the colon or rectum. It’s one of the most commonly diagnosed cancers worldwide, affecting around 1.9 million people each year.

We already know several factors that can increase the risk of developing bowel cancer. This includes older age, family history, smoking, obesity, alcohol and diets high in processed meat. But researchers are increasingly interested in another possible contributor: the trillions of microorganisms that live in our digestive system (our microbiome), and the infections that can alter this environment.

H pylori is an especially interesting candidate. Not only is the bacterium adapted to survive in the stomach’s acidic environment, it is also known to cause gastric cancer.

It’s thought around half of the world’s population are infected with H pylori– although rates vary greatly between countries. Infection often occurs during childhood.

Most infected people never develop symptoms. But in some, H pylori causes long-term inflammation of the........

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