Viral meningoencephalitis (St. Louis encephalitis virus): In viral meningoencephalitis the virus invades the parenchyma of the central nervous system. Viral destruction of neural elements evokes a microglial response. In this slide the clusters of small cells are almost entirely pleomorphic microglia (microglial nodules). Similar changes, varying in intensity, are seen in almost all forms of viral meningoencephalitis. Lymphocytic meningitis is usually also present.