Is it known what causes colitus and chrones disease?

Yes and no. We know what happens, but not why. Very briefly, ulcerative colitis (uc) and crohn's are distinct but similar, and involve dysregulation of the immune system in the gut. There are certain genetic risks, but we don't know for sure why the process happens in some people but not others, so in that sense we don't know what directly causes them. See http://tinyurl.Com/6u8bb for more info.
Not simple. There are a lot of different theories. However, it is thought to be related to an auto-immune process. That is, your natural defenses against infection attack your normal tissue. Whether it does this on it's own or triggered due to another invasive infection is unclear.