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Encephalitis is inflammation of the brain tissue. It's most often caused by viral infections. In some cases, bacterial infections can also cause encephalitis.
Encephalitis is most commonly due to viruses, such as herpes simplex, herpes zoster, cytomegalovirus, or West Nile virus. It can occur in the following ways: A virus directly infects the brain. A virus that caused an infection in the past becomes reactivated and directly damages the brain.