The analytical methods of nucleic acids are excellent tools to find virus antigens in various tissues, such as CSF, blood and brain ; polymerase chain reaction (PCR), restriction fragment length polymorphisms (RFLP) and hybridization techniques. Antigens can be isolated from cellular and/or supernatant fractions of the CSF of patients with common viral infections. The latter means the possible infection of virus in the nervous system, however, the former depends on the species of isolated cells. If the infected cells are the brain cells and/or endothelial cells of the ventricular system, viral infection in the nervous system is inferred. On the contrary, if they are blood cells, macrophages or monocytes, further studies of EGG, cytokine assay and other clinical and laboratory tests are necessary for the diagnosis of the nervous system infections.