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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.
The most common cause of Encephalitis is viral infection. The same Epstein-Barr virus which causes infectious mononucleosis is also one possible virus which can cause Encephalitis. Symptoms of encephalitis include double vision, hallucinations and difficulty looking at bright lights.