The heterogeneity of Crohn's disease suggests that it would be unwise to dismiss an infectious contribution to the pathogenesis in a subset of patients. The most enduring infectious candidate has been Mycobacterium paratuberculosis, which appears to be widespread in nature and appears to have the potential to infect humans. However, there are many counterarguments to the notion that MAP causes Crohn's disease, and numerous observations are seemingly at variance with this concept.