Encephalitis refers to infection of the brain parenchyma. The aetiology is due to a virus and the most important virus is Herpes simplex. The disease is characterised by extensive and asymmetric necrosis of the temporal lobes, as can be seen in this picture. Without treatment with antiviral drugs, the disease is rapidly fatal.

The disease can be diagnosed by a brain biopsy early in the onset examined by immunoflorescence.