One of the most lethal cancers can send loud warning signs to let you know something is wrong.

Actor Chadwick Boseman was 43 when he died of colon cancer last week, a high-profile reminder that it's not just an older person’s disease. More adults in their 20s and 30s are being diagnosed, a study published in the Journal of the National Cancer Institute found.

Even as screening has helped to lower the overall number of cases, the obesity epidemic may be fueling the rise among young adults.

Katie Couric’s husband Jay Monahan was just 42 when he died of colon cancer in 1998.

In recent years, TODAY's Craig Melvin learned his brother had been diagnosed with stage 4 colon cancer in his late 30s.

Doctors say it can be awkward for patients to discuss the symptoms.

“People are maybe sometimes uncomfortable about talking about that part of their body,” Dr. Jennifer Inra, a gastroenterologist at Brigham and Women's Hospital in Boston, told TODAY.

“There’s an awareness among the public, but not enough people are being screened… people are sometimes nervous about the screening tests.”

Colorectal cancer is the third most common cancer diagnosed in the U.S. and the third leading cause of cancer deaths in American men and women combined, according to the CDC.

Here are six symptoms you should never ignore:

1. Bleeding

Probably the most common warning sign is rectal bleeding, said Dr. Alfred Neugut, a medical oncologist and cancer epidemiologist at the Columbia University Mailman School of Public Health. If you notice blood on the toilet paper, in the toilet bowl or mixed in with your stool, tell your doctor. The blood can be bright red or a darker maroon color.

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It would generally be more significant bleeding than that caused by hemorrhoids or a cut in the area, Inra added.

“A lot of people don’t look at their stool and so it’s important to look. It’s important to see what’s going on,” she said.

If you notice blood, don’t ignore it.

“Rectal bleeding is something, believe it or not, people can ignore for very long periods of time,” Neugut said. “It can be intermittent, so you might have it one day and then it’ll go away for a few weeks and then you’ll get it again. So in-between, you’ll think you’re OK.” But you may not be.

2. Iron-deficiency anemia

When colon cancer tumors bleed, that causes iron loss in your body. People may not be aware that they’re losing blood, but a routine blood test will reveal anemia, or not having enough healthy red blood cells, Inra said.

3. Abdominal pain

A tumor could cause a blockage or a tear, causing cramps and other pain. The type of abdominal discomfort you may experience — whether dull or sharp — depends on what’s going on.

“A sharp, extremely tender abdomen would signify to us maybe there was a perforation,” Inra noted.

Pain may be a sign that things can’t pass through. You may also experience nausea and vomiting, and abdominal distention.

Doctors refer to this as a change in your stool caliber. If your stools are regularly much thinner than before, this may suggest a tumor in the colon, Inra said. Watch for other changes in your bowel habits, like constipation.

5. An unproductive urge to have a bowel movement

Tenesmus is the feeling that you have to empty your bowels, but when you try, no stool passes. This can be caused by a tumor that’s in your rectum, Inra noted.

6. Unexplained weight loss

This is always a reason to consider colon cancer or any cancer, in general. You seem to be eating enough, but the disease can change the way your body uses food and prevent you from being able to absorb all the nutrients, the National Cancer Institute noted.

When should you start getting screenings?

The American Cancer Society now recommends starting screening when you turn 45, if you’re at average risk for developing colon cancer; earlier, if you have a family history of the disease or other risk factors. Screening has made a huge impact in reducing the number of colon cancer cases, Neugut said.

There are different methods available, so talk with your primary care doctor or a gastroenterologist about which one would work for you.

Colonoscopy is the most commonly used screening test, Neugut said. After Couric underwent the procedure live on TODAY in 2000, doctors called the subsequent rise in testing the “Couric Effect."

You can also choose a flexible sigmoidoscopy, which is essentially a shortened version of a colonoscopy; or fecal testing, which can detect blood in your stool or DNA that may be shed by a colon tumor.

“There is no one best test. The best test is the one that a patient will do,” Inra noted.

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