Encephalitis is an inflammation of the brain, usually caused by a viral infection. Although rare, it is potentially life-threatening, and may lead to permanent brain damage or death.
Many different viruses can cause encephalitis, including the herpes simplex virus (HSV - which also causes cold sores) and enteroviruses. In some cases infections are caused by mosquito bites (e.g. Murray Valley encephalitis, also known as Australian encephalitis; equine encephalitis). A milder form of encephalitis can also accompany some of the common childhood diseases such as chicken pox, measles, mumps, and rubella (German measles).
Symptoms may include a high fever and headache, accompanied by a stiff neck, vomiting, light sensitivity, and convulsions. Patients may also be drowsy, or confused, and difficult to rouse. In babies, there may be a bulge in the fontanelle.
Many of these symptoms also occur in other illnesses (e.g. migraines), so the presence of fever is an important indication.
Information courtesy of Brain Foundation.