There are many infectious agents that may be responsible for meningoencephalitis. Historical data, clinical examination, analysis of cerebrospinal fluid and imaging often do not make it possible to retain initially a precise diagnostic orientation. In emergency, it is essential to seek the infectious causes imposing a specific treatment. Herpes simplex virus encephalitis is rare but it must be be always searched and a presumptive treatment must be started because of the spontaneously fatal evolution or heavy neurological sequelae. Hypothesis of Listeria meningoencephalitis being often impossible to eliminate from the start, anti-biotherapy must also be started immediately. Tuberculous meningoencephalitis must be evoked according to the ground. Other uncommon curable infectious causes are to be considered according to the context.