What Happens in Encephalitis?

Encephalitis is a serious, sometimes fatal condition. People often recover fully from the illness within a couple of weeks, but others, especially children and older people, may have permanent problems such as seizures, memory loss, personality changes, or brain damage.

The chance of dying from encephalitis or having permanent problems after recovery depends on what caused the encephalitis, how severe the illness is, how old the person is, and how quickly the illness develops.

• In cases of encephalitis that are caused by the mumps, measles, flu, chickenpox, or mononucleosis (Epstein-Barr) virus, most people recover within 1 to 2 weeks. Serious illness and death can occur, but this is rare.
• In the United States, the vast majority of people recover from encephalitis caused by mosquito-borne viruses. These types of encephalitis include St. Louis encephalitis, La Crosse encephalitis, western equine encephalitis, eastern equine encephalitis, and West Nile encephalitis. The death rate ranges from 1 to 33 out of 100 people in these types of encephalitis.¹
• Herpes simplex encephalitis has a death rate of 70% to 80% when it is not treated. But early treatment with acyclovir lowers the death rate to about 30%.² Among those who survive the most severe form of this disease, many have permanent neurological problems, including seizures, memory loss, and dementia.

In general, young children, older adults, and people who have impaired immune systems are at the greatest risk of serious illness, permanent neurological problems (such as seizures, memory loss, personality changes, or intellectual disability), or death as a result of encephalitis.