Viral infections are most likely triggering factors of autoimmune diseases, although a single vial infection is not sufficient to cause clinically evident autoimmune diseases. Any disease that profoundly alters the immune system may cause perturbed viral infections, thereby rendering otherwise refractory patients susceptible to autoimmune diseases. In this regard, drug-induced hypersensitivity syndrome (DIHS), a drug rash characterized by sequential reactivations of herpesviruses and the subsequent development of autoimmune diseases, offers a unique opportunity to investigate the mechanism of how autoimmunity is elicited after viral infections. Indeed, several autoimmune diseases have been reported to occur at intervals of several months to years after clinical resolution of DIHS. Two representative cases who developed autoimmune diseases three to four years after DIHS are shown. Our recent analyses of the kinetics of a developing disease have shown that fully functional FoxP3(+) regulatory T (Treg) cells are expanded at the acute stage thereby allowing viral reactivations but lose their suppressive function coincident with their contraction upon clinical resolution. The functional defect of Treg cells would be responsible for the subsequent development of autoimmune diseases. Patients with DIHS need close monitoring because of possible progression to autoimmune diseases even after the complete resolution.