A very small coccoid or rod-shaped Gram-negative bacterium belonging to an order (Rickettsiales) of the phylum Proteobacteria. With one exception, rickettsias are obligate parasites, being unable to reproduce outside the cells of their hosts. Rickettsias can infect such arthropods as ticks, fleas, lice, and mites, through which they can be transmitted to vertebrates, including humans. The group includes the causal agents of Q fever, Rocky Mountain spotted fever, and forms of typhus. The only genus that can be grown in culture outside host cells is Bartonella (Rochalimaea), which includes B. quintana, the causal agent of trench fever.