Colorado tick fever (CTF) is an acute febrile (often diphasic ) viral disease with infrequent rash. It is caused by the Colorado tick fever virus, a reovirus transmitted by the tick Dermacentor andersoni. For the first 2 weeks of disease, free virus can be isolated from the blood. This is followed by a period during which the virus circulates inside of erythropoietic cells. A nonspecific febrile illness is the most common manifestation, but the virus occasionally targets other organ systems: cases with prominent hepatic or CNS manifestations have been reported. Fever is nearly universally present. Other common symptoms are headache, myalgias, arthralgias and fatigue. Severe disseminated intravascular coagulation and thrombocytopenia have been recorded as causes of rare fatal cases.