Final answer:
Rickettsia is the Alphaproteobacteria responsible for Rocky Mountain spotted fever and typhus, both serious diseases transmitted by tick bites.
Step-by-step explanation:
The Alphaproteobacterium that causes Rocky Mountain spotted fever and typhus is Rickettsia. Rickettsia rickettsii, specifically, is an obligate intracellular parasite that is transmitted by the bite of ticks such as the American dog tick, the Rocky Mountain wood tick, and the brown dog tick. This pathogen cannot synthesize its own ATP and relies on a host's cells to fulfill its energy needs. As such, it represents a serious human pathogen noted for life-threatening conditions like meningoencephalitis.