acute, febrile viral infection usually transmitted to humans by the bite of the tick Dermacentor andersoni. The virus is classified as an orbivirus of the family Reoviridae, a grouping of viruses that is characterized by the lack of a lipid envelope and the presence of two protein coats. D. andersoni requires a vertebrate host for a part of its life cycle. The main mammalian reservoir of the virus is the golden-mantled ground squirrel, Citellus lateralis. The carrier tick is found chiefly in the western parts of the United States, notably in Colorado, and is most active in the late spring and summer.

A few days following tick exposure, the fever onset is abrupt, often with intolerance to light, headache, and prostrating weakness. Aching soon becomes generalized, especially in the muscles and joints. Abdominal pain and vomiting occur occasionally. The first attack lasts about two days. After a complete remission of all signs and symptoms lasting also about two days, there is in most cases a second attack that may be even more acute than the first. Except for the rare development of brain inflammation (encephalitis) in young children, however, recovery is usually uncomplicated, and there is lifelong immunity.