No, it's not from any kind of bacteria or virus, although when I first started gaining symptoms I had to go through assloads of blood, stool and urine tests to make sure that I didn't have some bacteria or virus. They really don't know a whole lot about it other than some treatments that help. Yes, I said "help" and not "work". First of all, the success of treatment will vary greatly from person to person. I'm lucky and right now I'm being treated with relative success by a drug that I can more that likely take safely for the rest of my life if I need to. However, some drugs, like prednazone, can cause liver failure, excessive acne, heart problems, etc.
Um, so yeah, the main thing it does to me is inflames my small intestines like mad. Sometimes I ulcerate and I'll get blood in my stool. I think I'm usually running about a pint low on blood because of this and I'm a super lightweight with ANY drugs (legal or not). Also, different things will cause flare ups for different people. For me, the big killer seems to be alcohol and caffeine. So REd Bull and Jager? Bad idea.
Anyhow, I could probably go on for pages here, but I'd rather not. You can read a lot about what they don't know about Crohn's at those links.