Encephalitis refers to an acute, usually diffuse, inflammatory process affecting the brain. The clinical hallmark of acute encephalitis is the triad of fever, headache, and altered mental status. The most common and important cause of encephalitis is the infection by a virus although other organisms can cause the disease. This article is a general overview of the most common viral encephalitides, divided into two families, Flavivirus and Alphavirus, and provides details about virus and RNA interference. More detailed descriptions of each viral family are provided below.