a term applied to illnesses resulting from infection by any virus, that was formerly in the Arbovirus group, pathogenic for humans, in which the only constant manifestation is fever; rash, lymphadenopathy, or arthralgia (alone or in combination) may occur in some people but not in others; some viruses may induce infections in which undifferentiated type fever is the only manifestation, whereas other viruses may induce in some people only undifferentiated fever, and in others similar fever followed by secondary manifestations, e.g., a hemorrhagic fever or encephalitis.