The first thing to understand about colitis disease is that it's a disease of your gastrointestinal system. In other words, it's a disease that's at least partially caused by - or at least effected by - what you eat. If you're eating crappy food, you're more likely to suffer from colitis disease. That's a simple fact, and it's one that you're best understanding right off the bat.
Technically, colitis disease is an inflammation of the colon. The colon is the very final part of the digestive system in humans. It's what absorbs water and salt for solid waste (feces) before they're evacuated from your body. Another name for the colon is the large intestine. But unlike its neighbor the small intestine, the colon doesn't actually absorb nutrients from the food you eat.
The number on cause of colitis disease in humans is poor blood supply to the colon. This is called ischemic colitis disease, and it is due to a narrowing of the arteries that carry blood to the organ. Think of it as being analogous to heart disease. Everyone knows that heart disease is caused by plaque build-up in the arteries that carry blood to the heart. Starve the organ of blood, and it suffers, eventually leading to cardiac arrest.
The same thing can happen to your colon if you have full blown colitis disease. So you can see why diet and exercise are an important part of avoiding colitis disease. If you're eating right and exercising, you're much, much less likely to suffer from atherlosclerosis (the scientific name for the narrowing of the arteries) and therefore much more likely to maintain a proper blood supply to the colon. Paradoxically, if you're suffering from this type of colitis disease, you will have bloody bowel movements.
There are other causes for colitis disease, however. A common one is either bacterial or viral infaction, which causes infectious colitis disease. All the food-born illnesses you've heard of are the cuprits, from E. Coli to Salmonella to good, old fashioned food poisoning. With infectious colitis disease, you will tend not to have bloody stools but instead have constant diarrhea. In either instance, you need to keep well hydrated, since colitis can sap your body of its water stores.
So in short, just make sure that the food you eat is clean in order to avoid infections, and healthy in order to keep your circulatory system working at top notch efficiency. These are the best ways to avoid colitis.