Infections of the brain, spinal cord, or meninges by single celled organisms of the subkingdom PROTOZOA. The central nervous system may be the primary or secondary site of protozoal infection. Examples of primary infections include cerebral amebiasis, granulomatous amebic encephalitis, primary amebic meningoencephalitis, and TRYPANOSOMIASIS, AFRICAN. Cerebral malaria, cerebral babesiosis, and chagasic meningoencephalitis are examples of secondary infections. These diseases may occur as OPPORTUNISTIC INFECTIONS or arise in immunocompetent hosts. (From Joynt, Clinical Neurology, 1998, Ch27, pp37-47).