Definition of Encephalitis:

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

See also: Meningitis

Causes, incidence, and risk factors:

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
• Insect bites
• 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
• Echovirus
• Poliovirus

Other viruses that can cause encephalitis include:

• Adenovirus
• Cytomegalovirus
• Eastern Equine Encephalitis Virus
• Epstein-Barr virus
• Herpes simplex infection
• Measles
• Mumps
• Rabies
• Rubella
• Varicella (chickenpox or shingles)
• West Nile virus

AIDS patients and others at high-risk can develop encephalitis due to parasites such as:

• Certain roundworms
• Cysticercosis
• Toxoplasmosis

Although most forms of encephalitis are caused by viruses, the condition may also be caused by bacterial diseases, such as:

• Lyme disease
• Syphilis
• Tuberculosis

Extremely rarely, an allergic reaction to vaccinations can cause encephalitis. Autoimmune disease and the effects of cancer can also cause encephalitis.

Encephalitis is uncommon. The elderly and infants are more vulnerable and may have a more severe case of the disease.