Stained using a fluorescent antibody (FA) technique, this photomicrograph revealed a positive result for this stained serum specimen from a suspected rubella patient. Rubella virus is classified as a Togavirus, genus Rubivirus. It is most closely related to group A arboviruses, such as eastern and western equine encephalitis viruses. It is an enveloped RNA virus, with a single antigenic type that does not cross-react with other members of the togavirus group. Rubella virus is relatively unstable and is inactivated by lipid solvents, trypsin, formalin, ultraviolet light, low pH, heat, and amantadine.