Apologies about the late response - I've been in the process of moving, and in this heat its taken a while!
@Vexillologist I'm so glad you offered to answer questions! (I actually might take you up on that for a character I'm planning)
Crohn's Disease is under the umbrella of Irritable Bowel Disease (IBD) along with Ulcerative Colitis (UC). Many times Irritable Bowel Syndrome (IBS) is lumped in with these, however IBS does not involve inflammation or a physiological basis. Basically Crohn's Disease is chronic inflammation in your GI tract that can occur anywhere along that route (mouth to anus). UC also has chronic inflammation, however it is only in the large intestine (colon), only affects the top layers, and does not affect other areas of the body, only resulting in symptoms such as abdominal pain, loose or bloody stools, fatigue, loss of appetite and potentially anemia. UC rarely if ever require surgery, and can go into remission easier and for longer periods of time.
Crohn's Disease is a more "serious" disease which affects the top layers as well as deeper layers of the GI tract. Mine is located in the Ilium, which is the small section that connects the small and large intestine. It's sort of like a consistent burn that results in scarring if not treated properly. Symptoms include bouncing between diarrhea and constipation, abdominal pain, fevers, rectal bleeding/bloody stools, and serious fatigue (similar to UC but usually more severe). Plus, because that section of the GI tract is inflamed and scarring, little to no nutrients can pass through the intestinal lining, which often results in vitamin deficiencies thus increasing fatigue. This also leads to skin, vision, joint and liver problems. Because of the scarring and inflammation, things like intestine blockages can occur, leading to ulcers, or fistulas which are holes in your digestive tract that causes fluids to leak into your skin or other organs. It can also lead to the fusing of two sections of intestines together. Finally, Crohn's Disease increases risk for colon cancer. I'm also on a very restrictive diet, as many foods will cause an increase in symptoms, it's called the FODMAP diet.
My "level" of severity is relatively average, bordering severe, as I have to be on basically super powered anti-inflammatory medicine taken four times a day, and even after three years have yet to get close to remission. Severe cases or long lasting cases can lead to surgery to cut out a section of intestine, but that doesn't necessarily guarantee remission (because even after surgery the disease doesn't just disappear). When I was first having issues and diagnosed, I could barely do the dishes for 20 minutes, and was in constant pain, especially after eating (as that's when symptoms are most severe), but now I can function moderately, aside from increasing vision issues, memory recall problems and general foggy head, as well as general weakness and fatigue. Basically I could possibly manage a part-time job (right now I'm working maybe 8 hours a week as an editor/office assistant, trying to find a part-time job I could do), and I can be active for around four days straight, but I will crash and hardly be able to do much for a few days to a couple weeks. When I have those "crashes" or have increased symptoms, its called a "flare up". During those times I can barely make myself a meal or take a shower. If you're interested in learning more about the lowered physical and mental capabilities of people with Crohn's/UC/IBD, and many others with immune diseases, check out the "Spoon Theory." It's been an incredibly helpful tool for me to explain what I'm capable of doing.
As to who is likely to get the disease, they have yet to find any definitive links, however the general demographic is people in urban areas and of higher socioeconomic brackets, industrialized countries, norther climates, Caucasians, and people who eat high-fat diets. Most people who are diagnosed are under the age of 30, but can be diagnosed later in life. There seems to be a correlation to genetics, but nothing firmly held. Right now there are no cures, and no definitive causes, however they believe it might be those with an overactive immune system, or have something to do with a certain "chemical" found in a small percentage of dairy cows (I'd have to find that article I read again to give more detail on that).
I hope that wasn't too long an introductory explanation! If you have any other questions feel free to ask! I ain't shy!