Scientists have a new clue in the mystery of why younger people are getting more colon cancer. It may have to do with a toxin produced by a common gut microbe.

Once considered a disease of older adults, colorectal cancer is now on the rise among young people in at least 27 countries. Its incidence in adults under 50 has roughly doubled every decade for the past 20 years.

Diagnoses have increased by 15% since 2004 among adults under 50. Scientists have struggled to find the reason why.