The rickettsial diseases are a group of related maladies with common characteristics such as arthropod vectors, obligate intracellular etiologic agents, and similar symptoms, including skin rashes, high fever, and headache. The prototype is classic, epidemic, louse-borne typhus fever. Most other rickettsial diseases were originally described as “typhus-like” and were differentiated from the classic disease during the twentieth century.

Those whose etiologic agents share the Rickettsia genus with the historic disease are murine, or fleaborne typhus, Rocky Mountain spotted fever and other members of the spotted fever group of diseases, and scrub typhus or tsutsugamushi. Two other diseases, Q fever and trench fever, are also known as rickettsial diseases. In recent decades, however, key differences in the clinical manifestations, in mode of transmission, and in the physiology of the etiologic agents of these two diseases have caused them to be placed in separate genera.

Pathological rickettsiae were discovered early in the twentieth century and named after Howard Taylor Ricketts, a University of Chicago investigator, who lost his life in research on typhus in Mexico after several years of fruitful research on Rocky Mountain spotted fever. Although smaller than most bacteria, rickettsiae are visible under the light microscope. Unlike common bacteria, they are obligate intracellular parasites – that is, they metabolize and multiply only inside living cells, a characteristic shared with the viruses. This peculiar combination of traits caused the rickettsiae to be classified for several decades as organisms midway between bacteria and viruses. By the late 1960s, however, research revealed that they were true, if highly fastidious, bacteria.