Definition of Encephalitis:

Encephalitis is irritation and swelling (inflammation) of the brain, most often due to infections.

See also: Meningitis

Causes, incidence, and risk factors:

Encephalitis is a rare condition. It occurs more often in the first year of life and decreases with age. The very young and the elderly are more likely to have a severe case.

Encephalitis is most often caused by a viral infection. Many types of viruses may cause it. Exposure to viruses can occur through:

• Breathing in respiratory droplets from an infected person
• Contaminated food or drink
• Mosquito, tick, and other insect bites
• Skin contact

Different viruses will occur in different locations. Many cases will tend to cluster in a certain season.

Encephalitis caused by the herpes simplex virus is the leading cause of more severe cases in all ages, including newborns.

A number of viruses for which there is now a vaccine may also cause encephalitis. These include:

• Measles
• Mumps
• Polio
• Rabies
• Rubella
• Varicella (chickenpox)

Other viruses that cause encephalitis include:

• Adenovirus
• Coxsackievirus
• Cytomegalovirus
• Eastern Equine Encephalitis Virus
• Echovirus
• West Nile virus

The virus causes inflammation of brain tissue. The brain tissue swells (cerebral edema), which may destroy nerve cells, cause bleeding in the brain (intracerebral hemorrhage), and brain damage.

Other causes of encephalitis may include:

• An allergic reaction to vaccinations
• Autoimmune disease
• Bacteria, such as Lyme disease,syphilis, and tuberculosis
• Parasites such as roundworms, cysticercosis, and toxoplasmosis in AIDS patients and other people who have a weakened immune system
• The effects of cancer