In the U.S., Dermacentor variabilis (the American dog tick or wood tick) and Dermacentor andersoni (the Rocky Mountain wood tick) transmits Rickettsia rickettsii (Rocky Mountain Spotted Fever) and Francisella tularensis (tularemia) to humans. D. andersoni also transmits Colorado tick fever virus. Both species can cause North American Tick paralysis, a flaccid paralytic condition caused by a neurotoxin in the tick's saliva.
As some of the readers mentioned, Ehrlichia chaffeensis and E. ewingii (as well as Anaplasma phagocytophilum) have also been found in D. variabilis, although this tick is not likely a common or effective vector of these bacteria.
As a side note, there are also Dermacentor ticks in Europe, Asia, and other parts of the world. In Europe, Dermacentor ticks are the likely vectors of Rickettsia slovaca and R. raoultii.
From Blaine Mathison:
A hunter returned after trapping for beaver
And found Dermacentor on his golden retriever!
some risks from the tick he could foresee,
The worst being Rickettsia ricketsii,
the causal agent of Rocky Mountain spotted fever