Viruses have evolved mechanisms enabling them to easily infiltrate the nervous system. Two main methods of viral entry are transneuronal and hematogenous spread. PCR is frequently used to for rapid identification of specific DNA viruses from the CSF, while Reverse transcriptase PCR is commonly used to identify RNA viruses in the CSF. Viral replication tends to peak early and then decline to undetectable levels in CNS infection. PCR assays have a higher incidence of detecting CNS viral infection.