A brain inflammation of viral or other microbial origin. Symptoms include headaches, neck pain, fever, nausea, vomiting, and nervous system problems. Several types of opportunistic infections can cause encephalitis.

A protein that initiates and controls a chemical reaction in the body or in a cell without itself becoming changed during the reaction.

A herpes-like virus that causes one of the two kinds of mononucleosis (the other is caused by CMV). It infects the nose and throat and is contagious. EBV lies dormant in the lymph glands and has been associated with Burkitt's lymphoma and hairy leukoplakia.