- any member of the genus Rickettsia, comprising rod-shaped to coccoid microorganisms that resemble bacteria but can be as small as a large virus and reproduce only inside a living cell, parasitic in fleas, ticks, lice, and mites and transmitted by bite to vertebrate hosts, including humans, causing such severe diseases as typhus and Rocky Mountain spotted fever.
- any rickettsia or rickettsialike microorganism of the orders Rickettsiales and Chlamydiales.
- any of a group of parasitic bacteria that live in the tissues of ticks, mites, and other arthropods, and cause disease when transmitted to man and other animals
