Structural similarity between a viral protein and a self-component can trigger an autoimmune response, which is the basis of molecular mimicry. Alternatively an invading virus can induce an inflammatory response which in turn can initiate an attack by hitherto dormant T cells on a specific self-antigen, a phenomenon which is referred to as Bystander Activation. Several viruses share amino acid sequences with target self-proteins. A widely studied viral interaction is the structural mimicry of a small portion of coxsackie virus to a specific region of the enzyme glutamic acid decarboxylase (GAD) which is expressed by the β cells of the islet of Langerhans in the pancreas leading to the destruction of insulin producing cells and the onset of Type I insulin dependent diabetes mellitus (IDDM). Knowledge of specific epitopes in GAD susceptible to autoimmune attack can permit devising therapeutic strategies for the prevention and suppression of IDDM.