Definition of Encephalitis:

Encephalitis is an inflammation (irritation and swelling) of the brain, usually caused by infections. See also meningitis.

Causes, incidence, and risk factors:

Encephalitis is most often caused by a viral infection, and many types of viruses may cause it. Exposure to viruses can occur through insect bites, food or drink contamination, inhalation of respiratory droplets from an infected person, or skin contact. In rural areas, arboviruses -- carried by mosquitoes or ticks, or accidentally ingested, are the most common cause.

In urban areas, enteroviruses are most common, including coxsackievirus, poliovirus, and echovirus. Other causes include herpes simplex infection, varicella (chickenpox or shingles), measles, mumps, rubella, adenovirus, rabies, Eastern Equine Encephalitis Virus, West Nile virus, and extremely rarely, allergic reaction to vaccinations.

Once the virus has entered the bloodstream, it may localize in the brain, causing inflammation of brain tissue and surrounding membranes. White blood cells invade the brain tissue as they try to fight off the infection.

The brain tissue swells (cerebral edema), which may cause destruction of nerve cells, bleeding within the brain (intracerebral hemorrhage), and brain damage.

Encephalitis is uncommon. It affects approximately 1,500 people per year in the U.S. The elderly and infants are more vulnerable and may have a more severe course of the disease.

Although most forms of encephalitis are caused by viruses, bacteria can also cause this problem. For example, Lyme disease, syphilis, and tuberculosis, all caused by bacteria, can cause encephalitis. AIDS patients and various other high risk individuals throughout the world can develop encephalitis due to parasites such as toxoplasmosis. Auto-immune disease and effects of cancer can also cause encephalitis.