James Van Der Beek has colorectal cancer – a shocking, scary diagnosis for anyone. And 47 years old is especially young for such a condition, right?

Not exactly. Colorectal cancer is one of 17 different types of cancer rising for Gen X and Millennials, according to American Cancer Society research published earlier this year. Breast, liver and ovarian cancer are also rising. Young celebrity after celebrity have recently announced they have (or had) cancer. Jenna Fischer. Olivia Munn. Princess Kate. Elle Macpherson. The list goes on.

What's going on? There's no exact cause known for the rise in cancers among young people, according to experts, but researchers are trying to figure it out. The best bet for a health-conscious person is fine-tuning diet and exercise routines, staying up-to-date on screenings, listening to your body and going to regular doctor's appointments.

Screenings will vary by age group. But given the higher incidence rate in young people, would that mean earlier screenings may be necessary? "We're not, at this point, recommending that everybody who's 30 go out and get screening tests," explains Dr. John Marshall, director of The Ruesch Center for the Cure of GI Cancers at Georgetown University, "but what we are recommending is that when people do have symptoms, that they don't write it off as other things."

'Out of the blue'

When Marshall – a self-described "old oncologist" – first started his career, no one under the age of 50 sat down in his clinic. Now they make up about half his patients. A significant portion are in their 30s and 40s, mostly with colon cancer.

"Most of these people are in fact, very healthy, very conscious of what they eat," he says. "They're exercise people, they're the 'right weight,' and no family history for these cancers, and it's kind of out of the blue."

He's not alone. "In my own practice I have seen, and statistically we have seen nationwide," says Dr. Emil Lou, professor of medicine at the University of Minnesota, "a stunning rise in cases that we now refer to as 'early onset' or 'young adult' cancers, generally defined as occurring in individuals younger than 50 years of age."

A leading theory behind this incidence uptick is all the changes to our microbiome over time – the environment, air, water, etc. "A lot of folks suspect that there's early life exposures that may have something to do with either our water supply or our food supply," says Dr. Mohamed Abazeed, co-leader of the Lung Cancer Program in the Lurie Cancer Center at Northwestern University.

Marshall wonders: Is it our water quality? Is it food processing? All these factors have potential consequences for our health, beyond cancer. But cancer is drawing attention given its grave nature.

"When we look at general population trends, it's almost impossible to say if it's factor A or factor B, or even factor A and B," adds Dr. Mike Varshavski, a family medicine physician and health influencer. "It's usually so complex and multifactorial." Still, he thinks "one of the major drivers of cancers, especially ones that are susceptible to it, come from the rising rates of obesity in the United States and across the globe."

Abazeed adds: "When you change somebody's diet dramatically, or you change the population's diet dramatically in such a short period of time, we haven't yet adapted to that, and the way our body reacts to that is by mounting an inflammatory response."

Did you see?Patrick Dempsey has 'anxiety' about cancer screenings. Yes, even Dr. McDreamy.

Screenings 'not just preventive'

Perhaps if people routinely visited their primary care physician, they could catch diseases before they develop or spread. More than three-quarters of Americans put off important health check-ups, according to a recent Aflac Wellness Matters Survey. It's higher for millennials, at 84%.

For those eligible for health screenings like a colonoscopy, physicians could catch and remove precancerous polyps in addition to checking for cancer already present.

"A lot of these procedures that we do in an office setting are not just preventive, they're proactive," Varshavski says.

Jenna Fischer, 50, said in an Instagram post sharing her breast cancer journey: "My tumor was so small it could not be felt on a physical exam. If I had waited six months longer, things could have been much worse. It could have spread."

Katie Couric:Colon cancer awareness, breast cancer diagnosis and becoming a grandmother

Treat your symptoms 'seriously'

Of course, it's critical to think about data in context. People have been less likely to die from cancer over the last few decades despite the increasing incidence rate.

Still, Marshall cautious both patients and medical professionals to not brush away concerning symptoms just because someone is young. Don't send a 30-year-old with rectal bleeding away assuming it's a hemorrhoid; if that person was 60 or 70, you'd immediately rule out colon cancer. "We need to change the tradition," he says.

Lou adds: "Ultimately, knowing your body and trusting your intuition if you feel something is wrong is important to recognize at any age. If you report your symptoms to a medical care team and don’t feel that you are being heard, then be persistent and seek out additional opinions."

The short of it: If you experience dietary or weight changes or a shift in bowel habits that lasts for weeks? Go to the doctor. "That's generally the way these things show up," Marshall says, "sort of vague, early symptoms that don't seem to go away. Treat them seriously."