recover from viral encephalitis without long-term problems related to the illness. Mortality rates vary for those who do not receive treatment, for example being about 70% for herpes encephalitis but low for the La Crosse virus. Individuals who remain symptomatic after initial infection may have difficulty concentrating, behavior or speech disorders, or memory loss. Rarely, individuals may remain in a persistent vegetative state. The most common long-term complication of viral encephalitis is seizures that may occur in 10% to 20% of patients over several decades. These seizures are resistant to medical therapy. However, individuals who have unilateral mesial temporal lobe seizures after viral encephalitis have good results following neurosurgery. Prognoses related to specific viruses include: For Eastern equine encephalitis, some children may experience seizures, severe intellectual disability, and various forms of paralysis. For Japanese encephalitis, extrapyramidal symptoms relating to motor function may remain. For St. Louis encephalitis, low blood sodium level and excess, unsuppressable release of antidiuretic hormone For Western equine encephalitis, some children may experience seizures and behavioral changes. For pregnant women infected with Zika virus, the newborn child may have microcephaly. Other potential complications following viral encephalitis include: Encephalopathy Flaccid paralysis Impaired intelligence Low blood sodium level Mononeuropathy Mood and behavioral changes Residual neurological deficits Epidemiology While the etiology of many cases of encephalitis is unknown, viruses account for about 70% of confirmed encephalitis cases, with the herpes simplex virus being the most common cause at about 50% of encephalitis cases. The incidence of viral encephalitis is about 3.5 to 7.5 per 100,000 people, with the highest incidence among the young and the elderly. Viral encephalitis caused by some viruses, such as the measles virus and the mumps virus, has become less common due to widespread vaccination. For others, such as Epstein-Barr virus and cytomegalovirus, incidence has increased due to the increased prevalence of AIDS, organ transplantation, and chemotherapy, which have increased the number of immunocompromised people who have weakened immune systems or who are susceptible to opportunistic infections. Time of the year, geography, and animal, including insect, exposure are also important. For example, arbovirus infections are seasonal and cause viral encephalitis at the highest rate during the summer and early fall when mosquitos are most active. Similarly, those who live in warm, humid climates where there are more mosquitos are more likely to experience viral encephalitis. Prevention As many encephalitic viruses are transmitted by mosquitos, many prevention