Causes of encephalitis
The exact cause of encephalitis is often unknown. Autoimmune inflammation, viral and bacterial infections, and noninfectious inflammatory conditions all can cause encephalitis. There are two main types of encephalitis: Primary encephalitis. This condition occurs when a virus or other agent directly infects the brain. The infection may be concentrated in one area or widespread. A primary infection may be a reactivation of a virus that had been inactive after a previous illness. Secondary encephalitis. This condition results from a faulty immune system reaction to an infection elsewhere in the body. Instead of attacking only the cells causing the infection, the immune system mistakenly attacks healthy cells in the brain. Also known as post-infection encephalitis, secondary encephalitis often occurs 2 to 3 weeks after the initial infection.