People become infected with Colorado tick fever (CTF) virus from the bite of an infected Rocky Mountain wood tick (Dermacentor andersoni). Rocky mountain wood ticks are found in the western United States and western Canada at 4,000–10,000 feet above sea level.
Ticks become infected with CTF virus when they feed on the blood of a reservoir animal (an animal that has the virus circulating in its bloodstream). The most important reservoirs for CTF virus are small rodents such as squirrels, chipmunks, and mice. After the tick becomes infected, it passes the virus to other hosts (animal or human) while it feeds.
The virus is not transmitted from person to person, except in rare instances by blood transfusion. Since CTF virus can remain in red blood cells for several months, blood and bone marrow should not be donated for 6 months following infection.