How this disease is contracted is not really known but it is suspected that hidden triggers such as viruses, bacteria, genetics and some drugs may be linked. Viruses that seem to play a greater role in susceptibility are:

Herpesvirus 6 (HHV6)
Epstein-Barr Virus
Cytomegalovirus


The main bacterial suspect is mycobacterium para tuberculosis, it has been found to cause severe enteritis in many animals, 2/3 had the bacterium present. With genetics it was determined that those with the disease present in the family were 3.5 to 10% more likely to contract the disease.
NSAIDS - (non steroidal anti-inflammatory drugs) - 38% of those tested were found to have this drug in their system or had been taking these particular type of drugs. More research is being done, Professor John Hermon-Taylor at St. George Medical School found that 1 out of every 4 samples of whole milk purchased from retailers had the disease causing bacterium present suggesting that the pasteurizing process was not 100% affective.
Dr. Andrew Wakefield, Royal Free Hospital in London recently suggested that this disease might not even be an ulcerative disease but was caused by restricted blood flow in the vessels that supply the gut and that these retrictions are possibly caused by small tumors created by a persistent measles virus.

Contracted

(aka regional enteritis)
Crohn's disease was first described by Dr. Burrill B. Crohn along with Dr. Leon Ginzburg and Dr. Gordon D. Oppenheimer in 1932. This disease belongs to a group of Irritable Bowel Diseases (IBD) and is chronic inflammation of the gastrointestinal tract that may affect any part of the tract from the mouth to the anus.

CROHN'S DISEASE