Researchers led by the University of California, San Diego, have identified a potential source driving the rising rates of colorectal cancer among young adults.

Colorectal cancer is one of the deadliest cancers in both men and women, surpassed only by breast cancer and lung cancer. Now, researchers say they may have stumbled upon the reason why the deadly disease is seeing a surge in diagnoses among young adults.

The researchers say the bacterial toxin colibactin, produced by certain strains of E. coli, could be causing mutations in the colon’s DNA. These mutations, say the team, can increase a person’s risk of developing cancer before age 50.

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Despite starting their research examining colorectal cancer more broadly, the team ultimately found a difference in cancer cells between younger and older individuals.

Ludmil Alexandrov, lead author of the study and professor in the Department of Cellular and Molecular Medicine at UC San Diego, said he and his team “were very surprised to see that there was a bacteria, and that bacteria was generating mutations.”

“When we started this study, we weren’t looking to answer this specific question,” said Alexandrov, per Fortune.

While some strains of E. coli can make you sick, some do not cause any symptoms. As a result, many people may be unaware that they live with the bacteria in their systems. Because of this, it is challenging to know if the toxic colibactin entered your gut. Moreover, exposure to the toxin does not necessarily mean you will develop cancer.

Alexandrov theorizes that certain societal changes may be contributing to younger people’s higher odds of exposure to E. coli and colibactin, such as the higher usage of cesarean sections. Higher rates of antibiotic use may also be contributing, as might the drop in breastfeeding and the abundance of processed foods in our diets.

Alexandrov hopes the team’s findings could lead to the development of a stool test to identify if people have had the infection and mutations, helping identify people at risk of colorectal cancer earlier.