Crohn's disease is an inflammatory disorder of the gastrointestinal tract, the cause of which remains unknown. Since the first description by Dalziel in 1913 (1), similarities between Crohn's disease and intestinal mycobacterial infection, particularly Johne's disease in ruminants, have been widely recognized (2, 3). After Mitchell and Rees demonstrated the transmission of granulomata from Crohn's disease by injecting intestinal homogenates into the footpads of mice (4), there followed many studies attempting to identify infective agents within the bowel of patients with Crohn's disease. Although Mycobacterium paratuberculosis has been identified in intestinal tissue from a proportion of patients with Crohn's disease, a convincing role for this agent in the aetiology of Crohn's disease has not been established. (5). Likewise, extensive studies into bacterial (6-9) and viral (10) agents potentially associated with Crohn's disease have been inconclusive, although recent ultrastructural observations of viral particles within submucosal granulomata have added a new impetus to the search (11, 12). This review examines the evidence for an association between Crohn's disease and viral infection from epidemiological studies, transmission and cell culture, specific immunological responses, ultrastructure and from molecular biological techniques.
