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In the world of the other CD (Crohn's disease) which is also auto-immune but
without a known cause, there is ancedotal evidence that quitting smoking after
many years can bring on the first occurance of Crohn's or Ulcerative Colitis.
This has happened to 2 of my male cousins in their late 30's.
 
One explanation is that smoking depresses the immune system and stopping
smoking strengthens it to the point that it reacts inappropriately to certain
stimuli.  One treatment for Crohns and Colitis is medication to weaken (just
enough) the immune system without succumbing to opportunistic infections.
 
It's one theory?!