The rapid diagnosis of viral infection in nervous system is necessary for the effective treatment, since they progress so rapidly. The identification of infected virus can be achieved by amplifying DNA or RNA in virus, using polymerase chain reaction (PCR). Herpes simplex encephalitis is the most common and fruitful target for genetic diagnosis. Genetic diagnosis can also detect the presence of cytomegalovirus, EB virus, human herpes 6 virus, herpes zoster virus, HTLV-1 (human T-lymphotrophic virus type 1), measles virus, mumps virus, Japanese encephalitis virus, rubella virus, HIV (human immunodeficiency virus), and HCV (hepatitis C virus). However, the presence of the virus does not always mean a recent infection by the virus, nor a cause of the disease.