Herpes simplex type 1 virus resides in the Vth nerve Gasserian ganglion and can present intermittently as so-called cold sores or apthous ulcers around the lips and mouth. Rarely it can pass retrogradely into the brainstem via the root entry zone to produce a rhombencephalitis as in this case leading to marked (V1, 2 and 3 sensory symptoms). MRI features are of focal inflammation and disruption to blood-brain and blood-cranial nerve barrier (Gd-enhancement). It is usually self-limiting and should not be confused with acute demyelination. Involvement of the Vth cranial nerve is key to this differentiation. The rapidity of onset, presence of cold sores and absence of cancer history discriminate from perineural spread of head and neck neoplasm.