Why are so many younger Americans getting and dying of colorectal cancer?

Research shows many adults in their 20s, 30s and 40s are being diagnosed with colorectal cancer, with incidence increasing over the last three decades.

Colorectal cancer was once viewed as being mostly diagnosed among middle-age and older adults, but that's changing.

Research shows more adults in their 20s, 30s and 40s are being diagnosed with colorectal cancer, with incidence increasing over the last three decades.

Deaths are on the rise too, with a recent study finding colorectal cancer is now the leading cause of cancer-related deaths in men -- and second in women, under age 50.

"It's definitely incredibly concerning that these rates continue to rise and that we really don't know why this is happening," Dr. Andrea Cercek, a gastrointestinal medical oncologist at Memorial Sloan Kettering Cancer Center in New York City, told ABC News.

Oncologists told ABC News that it's important to get the word out about how colorectal cancer is affecting younger adults in an effort to try and reverse the trends.