Herpes Simplex Virus encephalitis is the most common infectious encephalitis, with an estimated annual incidence of 1 / 250,000 to 1 / 500,000 in industrialized countries. Despite a widely used antiviral treatment, the prognosis remains poor with a mortality of 5 to 20% and a considerable morbidity rate. One of the contributing factors of bad prognosis is the development of encephalitis mediated by autoantibodies, most often directed against NMDA receptors, in the weeks following viral encephalitis. The description of this pathology is recent, the pathophysiology of this process remains poorly understood, and the management of these patients is not yet codified.