The definition of the Rickettsiaceae family has changed since the mid-1980s on the basis of new genetic tools, allowing the discovery of new species and the reclassification of other species. The Rickettsiaceae are aerobic, small, gram-negative coccobacilli that are obligate intracellular parasites of eukaryotic cells. The Rickettsiales now consist of the genus Rickettsia, the new genus Orientia, and the Ehrlichia group. There are three groups of Rickettsiae based on clinical presentation: the spotted fever group, the typhus group, and scrub typhus. Coxiella burnetii, the agent that causes Q fever, is covered in this chapter but is now classified in the γ-proteobacteria group, together with Legionella and Francisella tularensis.

The pathogenesis of illness due to the Rickettsiae is vasculitis. The rickettsiae proliferate in the endothelial lining cells of the small arteries, capillaries, and veins. Erhlichiae invade and proliferate in their target cells of the hematopoietic and lymphoreticular systems, whereas in Q fever the organisms are inhaled and proliferate in the lungs, causing inflammation and bacteremic seeding of other organs, particularly the liver. All of the important rickettsial infections are vectorborne, whereas Q fever usually results from inhalation of dust contaminated by the birth fluids of domestic ungulates.