Molecular mimicry is the process by which T cells activated in response to determinants on an infecting microorganism cross-react with self epitopes, leading to an autoimmune disease. Normally, infection of SJL/J mice with the BeAn strain of Theiler’s murine encephalomyelitis virus (TMEV) results in a persistent CNS infection, leading to a chronic progressive, CD4+ T cell-mediated demyelinating disease. Myelin damage is initiated by T cell responses to virus persisting in CNS APCs, and progressive demyelinating disease (50 days postinfection) is perpetuated by myelin epitope-specific CD4+ T cells activated by epitope spreading. We developed an infectious model of molecular mimicry by inserting a sequence encompassing the immunodominant myelin epitope, proteolipid protein (PLP) 139–151, into the coding region of a nonpathogenic TMEV variant. PLP139-TMEV-infected mice developed a rapid onset paralytic inflammatory, demyelinating disease paralleled by the activation of PLP139–151-specific CD4+ Th1 responses within 10–14 days postinfection. The current studies demonstrate that the early onset demyelinating disease induced by PLP139-TMEV is the direct result of autoreactive PLP139–151-specific CD4+ T cell responses. PLP139–151-specific CD4+ T cells from PLP139-TMEV-infected mice transferred demyelinating disease to naive recipients and PLP139–151-specific tolerance before infection prevented clinical disease. Finally, infection with the mimic virus at sites peripheral to the CNS induced early demyelinating disease, suggesting that the PLP139–151-specific CD4+ T cells could be activated in the periphery and traffic to the CNS. Collectively, infection with PLP139–151 mimic encoding TMEV serves as an excellent model for molecular mimicry by inducing pathologic myelin-specific CD4+ T cells via a natural virus infection.