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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.
Other viruses that can cause encephalitis include several related viruses: varicella-zoster virus (the cause of chickenpox and shingles), cytomegalovirus, Epstein-Barr virus (the most common cause of infectious mononucleosis) and human herpesvirus-6 (a cause of transient encephalitis in very young children).