A molecular approach to the diagnosis of virus infections.

Virus diseases are usually diagnosed by serological methods, provided specific antisera are available. With a "new" virus, however, knowledge of its physical and chemical properties is necessary before it can be assigned to one of the existing families. If a specimen contains sufficient virus, electron microscopy is the most valuable first test to apply because this often allows the agent to be placed immediately in a virus family. Further characterization can be achieved by measuring the stability of the virus in lipid solvents. With unenveloped viruses, the density in cesium chloride is a valuable diagnostic property. A more exact characterization can be made by examining the nucleic acid and protein of the virus. RNA viruses, for example, can be identified with great precision using base sequence homology and ribonuclease T1 mapping. Similar precision can be achieved with DNA viruses by the use of restriction endonucleases. Examination of the proteins of both RNA and DNA viruses also allows similar viruses to be distinguished. These methods have proved valuable in the epidemiology of those diseases for which serological methods did not allow the isolates to be identified with certainty.