Encephalitis is most often caused by a viral infection and many types of viruses can 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 that are accidentally ingested, are the most common cause.

In urban areas, enteroviruses are most common, including Coxsackie virus, poliovirus, and echovirus. Other causes include herpes simplex infection, varicella (chickenpox or shingles), measles, mumps, rubella, adenovirus, rabies, West Nile virus, and extremely rarely -- vaccinations.

Once the virus has entered the blood stream, it can localize in the brain causing inflammation of the 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) and can 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.