IIRC, regular TB become ingested by macrophages, but for whatever reason, the macrophage's enzymes are unable to digest the bacterium.
Cyborg found this interesting thread on the apparent association between Crohn's disease and Mycobacteria paratuberculosis.
If you want to try PubMed, start with Crohn's disease AND Mycobacteria paratuberculosis. The capitalized AND is functioning as a Boolean operator. Copy and paste Crohn's disease AND Mycobacteria paratuberculosis. Then just click on go. You might want to check articles with abstracts as a minimum.