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We have greatly increased our understanding of the genetics of inflammatory bowel disease (IBD) in the last decade; however, migrant studies highlight the importance of environment in disease risk. The possibility that IBD is an infection has been debated since the first description of Crohn’s disease. Mycobacterium avium paratuberculosis was the first organism to be suggested as an IBD pathogen, and it has been argued that it fulfils Koch’s postulates and could be designated the cause of Crohn’s disease. Other organisms have been postulated as possible IBD pathogens, including various Helicobacter species, one of which has been identified in primate colitis; others are widely used in animal models of IBD. Adherent-invasive Escherichia coli appear specific to ileal Crohn’s disease and have been shown to induce the release of TNF-α, a key cytokine in IBD inflammation. The aim of this article is to give …