Causes

The exact cause of encephalitis is often unknown. But when a cause is known, the most common is a viral infection. Bacterial infections and noninfectious inflammatory conditions also can cause encephalitis.

There are two main types of encephalitis:

• Primary encephalitis. This condition occurs when a virus or other agent directly infects the brain. The infection may be concentrated in one area or widespread. A primary infection may be a reactivation of a virus that had been inactive after a previous illness.
• Secondary encephalitis. This condition results from a faulty immune system reaction to an infection elsewhere in the body. Instead of attacking only the cells causing the infection, the immune system also mistakenly attacks healthy cells in the brain. Also known as post-infection encephalitis, secondary encephalitis often occurs two to three weeks after the initial infection.

Common viral causes

The viruses that can cause encephalitis include:

• Herpes simplex virus (HSV). Both HSV type 1 — associated with cold sores and fever blisters around your mouth — and HSV type 2 — associated with genital herpes — can cause encephalitis. Encephalitis caused by HSV type 1 is rare but can result in significant brain damage or death.
• Other herpes viruses. These include the Epstein-Barr virus, which commonly causes infectious mononucleosis, and the varicella-zoster virus, which commonly causes chickenpox and shingles.
• Enteroviruses. These viruses include the poliovirus and the coxsackievirus, which usually cause an illness with flu-like symptoms, eye inflammation and abdominal pain.
• Mosquito-borne viruses. These viruses can cause infections such as West Nile, La Crosse, St. Louis, western equine and eastern equine encephalitis. Symptoms of an infection might appear within a few days to a couple of weeks after exposure to a mosquito-borne virus.
• Tick-borne viruses. The Powassan virus is carried by ticks and causes encephalitis in the Midwestern United States. Symptoms usually appear about a week after a bite from an infected tick.
• Rabies virus. Infection with the rabies virus, which is usually transmitted by a bite from an infected animal, causes a rapid progression to encephalitis once symptoms begin. Rabies is a rare cause of encephalitis in the United States.
• Childhood infections. Common childhood infections — such as measles (rubeola), mumps and German measles (rubella) — used to be fairly common causes of secondary encephalitis. These causes are now rare in the United States due to the availability of vaccinations for these diseases.