Inflammatory bowel disease, a collective term for ulcerative colitis and Crohn's disease, is a chronic, immune-mediated disease of the gastrointestinal tract that develops in genetically susceptible individuals. The role of infection in the development of inflammatory bowel disease is underscored by various clinical observations, such as the delayed age of onset, suggesting that childhood exposure to pathogens is essential, and the clinical improvement that follows decreasing bacterial intestinal load. Over the years, many a pathogen has been linked to the development and exacerbation of inflammatory bowel disease, notably; Mycobacterium paratuberculosis, Escherichia coli, Listeria monocytogenes and Chlamydia as well as viruses such as measles, mumps, rubella, Epstein-Barr virus and cytomegalovirus. Presently, leading theories of disease pathogenesis suggest loss of immune tolerance to normal commensal bacteria coupled with excessive exposure to bacterial antigenic products. This review describes the most commonly implicated pathogens in the causation of IBD and presents the evidence supporting their pathogenic role as well as data that offset their importance.