Herpes simplex encephalitis is a rare neurological condition that is characterized by inflammation of the brain (encephalitis). People affected by this condition may experience a headache and fever for up to 5 days, followed by personality and behavioral changes; seizures; hallucinations; and altered levels of consciousness. Herpes simplex encephalitis is caused by a virus called the herpes simplex virus. Most cases are associated with herpes simplex virus type I (the cause of cold sores or fever blisters), although rare cases can be caused by herpes simplex virus type II (genital herpes). It is poorly understood why some people who are infected with herpes simplex virus develop Herpes simplex encephalitis while others do not. Changes in genes such as TLR3 and TRAF3 have been observed suggesting there may be a genetic component in some cases.