ABSTRACT
The order Rickettsiales comprises small rod-shaped or coccoid organisms parasitic of man or animals. They are considered Gram-negative, although the Gram stain is not the stain of choice. Some are obligate intracellular parasites, others may grow in a cell-free environment. Rickettsiales have both RNA and DNA. They exist in natural vertebrate hosts and are usually transmitted from host to host by more or less specific arthropod vectors. Most are capable of causing inapparent infections in their vertebrate and arthropod hosts.
