Experts are stumped as to why colorectal cancer is rising dramatically among people in their 30s and 40s.

Colorectal cancer is the deadliest cancer for men under age 50 — and the second deadliest cancer among women in the same age group, behind breast cancer.

The incidence of colon cancer has been rising for at least the last two decades, when it was the fourth-leading cause of cancer death for both men and women under 50.

Among men and women of all ages, lung cancer remains the leading cause of cancer death. Prostate cancer is second for men, and breast cancer is second for women. Colorectal cancer is third, overall, for both sexes.