The infectious hypothesis for IBD is certainly not new. The evidence for an infectious agent is tantalizing but not yet convincing. A failure to demonstrate a specific agent in the inflammatory process could mean that many different organisms are involved, or several organisms that, by themselves, do not cause disease interact. All candidate organisms proposed thus far are compatible with either hypothesis. Nevertheless, interaction between an agent(s) in a host modified by immunologic and genetic factors is still an attractive hypothesis for the cause of these illnesses.