Several species of Rickettsia can cause illnesses in humans.  These agents are usually not transmissible directly from person to person. Transmission occurs via an infected arthropod vector or through exposure to an infected animal reservoir host. In addition, transmission has been documented to occur via blood transfusion. Rickettsial agents that cause human disease are typically categorized not by disease manifestation but according to antigenic similarity. The clinical severity and duration of illnesses associated with different rickettsial infections vary considerably, even within a given antigenic group. Rickettsioses range in severity from diseases that are relatively mild (rickettsialpox) to those that can be life-threatening (Rocky Mountain spotted fever), and in duration from those that can be self-limiting (cat-scratch disease) to chronic (Q fever). Most patients with rickettsial infections recover with timely application of appropriate antibiotic therapy.