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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 can develop as a result of a direct infection to the brain by a virus, bacterium or fungus (infectious encephalitis, or primary encephalitis), or when the immune system responds to a previous infection; the immune system mistakenly attacks brain tissue (secondary encephalitis, or post-infectious encephalitis).