Rickettsiae are obligate intracellular bacteria that have evolved in association with arthropod vectors. The genus can be subdivided into typhus group and spotted fever group rickettsioses. These infections are present throughout the world. In the United States, the most common rickettsioses are Rocky Mountain spotted fever, caused by Rickettsia rickettsii and transmitted by tick vectors; and endemic typhus, caused by R. typhi and transmitted by fleas. Isolated outbreaks of rickettsial pox, caused by R. akari and transmitted by mites, have been described in New York. A spotted fever rickettsiosis caused by R. parkeri has been documented in the United States and Latin America since 2002.

All rickettsial infections target the endothelium lining the microvasculature; therefore, clinical manifestations are systemic, and the morbidity and mortality of the disease are due to cerebral and pulmonary edema resulting from increased microvascular permeability. In the lungs, the clinical picture is that of a noncardiogenic pulmonary edema and/or adult respiratory distress syndrome.