While encephalitis is rare in the United States, when it does occur it is usually caused by the herpes simplex virus.¹

Symptoms typical of herpes simplex encephalitis (HSE) that are less likely to occur in other types of encephalitis include speech problems, a reduced sense of smell, and partial seizures. Fever and headache occur as in other types of encephalitis. HSE usually can be diagnosed through analysis of spinal fluid obtained through a lumbar puncture (spinal tap).

In cases of HSE, early treatment (before the person becomes unconscious) with an antiviral medicine called acyclovir greatly increases the possibility of survival and recovery.¹

When HSE is not treated, it has a death rate of 70% to 80%.² Of people who survive the disease, many have permanent neurological problems, including seizures, memory loss, and dementia.