Colorectal cancer is often thought to affect older people, but one in five cases diagnosed today occurs in people younger than age 55, compared to one in 10 cases in 1995, according to a recent study published by the American Cancer Society.

There’s no clear explanation for this trend, but a new paper just published in Science suggests a number of possible reasons, including environmental and genetic factors. Low screening rates and misdiagnosis in people who don’t suspect cancer likely play a role as well.

“We're coming to a point where we shouldn't consider colorectal cancer a disease of only older adults,” said Andrew Chan, a professor of medicine at Harvard Medical School and vice chair