Encephalitis is an inflammatory condition within the brain tissues. It differs from meningitis, which is inflammation of the covering of the brain. The most common cause of this inflammation is an infection due to a virus, bacteria or fungus. Encephalitis can be a life-threatening condition; however, therapy with antibiotics and anti-inflammatory agents is often extremely effective. The severity and speed at which the inflammation develops impact on the seriousness of any neurological symptoms that may occur.

Headache is only one of these symptoms. The headache due to encephalitis may be extremely severe, lasting for a considerable time after signs an active inflammatory process has resolved.