The order Rickettsiales includes a diverse group of microorganisms. The criteria used to define genera (Rickettsia and Coxiella) within the order include morphology, association with various arthropods (primarily ticks, fleas, mites or lice), obligate intracellular parasitism, and serologic relatedness. Rickettsia, Coxiella and species within those genera have adapted to existence within the various arthropods and also frequently infect humans and other vertebrates, usually as accidental hosts. The table below summarizes the various rickettsioses, their respective etiologic agents, and their salient epidemiologic features.

Coxiella burnetii, the etiologic agent of Q fever, is distributed worldwide. It is transmitted to small wild animals, sheep, goats,cattle and dogs by ticks. Humans are infected by inhalation of infectious aerosol. Q fever is an occupational illness and usually takes the form of an acute systemic disease. The disease is most often characterized by the sudden onset of symptoms which may include malaise, chills and fever, myalgia and most characteristically, severe headache. An atypical pneumonia of unknown etiology with dry, unproductive cough, and slight chest pains may also be symptomatic of Q fever, but can be misdiagnosed as influenza. The onset of the disease in humans is 1-2 weeks from time of exposure, although it may be longer in elderly patients. The infection can be severe, leading to chronic disease characterized by hepatitis and endocarditis. During the peak of infection large numbers of microorganisms are shed in urine, feces, milk, placental tissue and amniotic fluid. Treatment with tetracycline, chloramphenicol, and rifampicin is effective in the early stages of the disease. Antibiotic treatment of the chronic disease or late stages of the acute disease requires a long-term regimen of chemoprophylaxis.