Type 1 diabetes (T1DM) is an autoimmune disease having complex genetic inheritance and proposed environmental factors contributing to its etiology. Newly recognized genetic susceptibilities for TIDM also confer susceptibility to mycobacterial infection. Mycobacterium avium subsp. paratuberculosis (MAP) is the cause of bovine Johne's disease and also is thought to act as an immune antigen in Crohn's disease and other granulomatous diseases. The observation that early exposure to cow's milk increases the risk of acquiring T1DM in the genetically at risk has spawned a large study, the TRIGR study. This paper postulates a causative role for MAP in T1DM in the genetically at risk.