Herpes encephalitis is a rare form of encephalitis caused by either of the two forms of the herpes virus. Herpes encephalitis is usually a more serious condition than encephalitis caused by other, rarer infections.

It is unclear why a common infection like herpes (which infects about 90% of adults in the United States) will, in about 1 out of 500,000 infected persons each year, cause life-threatening encephalitis. Through a mechanism that remains unclear, it is known that the herpes virus can migrate from the original infection site (usually the lips) through an axial nerve to the brain. There, it usually affects the temporal lobe, causing the resulting confusion and disorientation typical of the disease.

Diagnosis is through a lumbar puncture as the infection will appear in the cerebro-spinal fluid. Treatment is with high doses of acyclovir. However, even this treatment is ineffective in about a third of cases. In addition, it is uncommon for a survivor of the disease to regain full brain function, and many patients suffer serious permanent cognitive difficulties. Early treatment makes for a much better prognosis.

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