Encephalitis describes any condition where an infection causes acute inflammation of the tissues of the brain. Once common, it is becoming increasingly rare as the diseases that can lead to it are generally vaccinated against or become easier to treat. The patients in the movie Awakenings were all victims of a now rare type of encephalitis, encephalitis lethargica.
Some of the diseases that can cause encephalitis are Syphilis, toxoplasmosis, malaria, meningitis and herpes. The disease is actually the result of the immune system of the brain reacting to the infective agent and all the symptoms can be traced to this reaction.
The disease can only be treated by treating the underlying illness.
See also the article on herpes encephalitis.