Q fever is caused by the rickettsia Coxiella burnetti. Most human infection arises from direct or indirect association with cattle, sheep or goats and other wild animals which are the natural hosts. There are numerous tick hosts which act as the agent of transmission.
The human disease manifests as fever with severe headache behind the eyes, profuse sweating, myalgia, and nausea. Pulmonary involvement often occurs. Untreated it can be fatal at a rate of 2.4 percent for the acute form, and 60 percent if chronic endocarditis develops. There is evidence that the disease is also asymptomatic in many cases.
The disease occurs in nearly every country in the world. In some rural areas in Africa and the Middle East, and probably elsewhere, infection is almost universal in young children; adults there are thus immune and the acute illness is seen only in visitors or migrants.
Rickettsiae may be present on soil and dust, where they can survive for long periods, since they are highly resistant to drying. This is particularly important in those countries where ewes are brought into yards to lamb. Very high concentrations of rickettsiae may then be present in the dust of such yards, which is therefore highly infective when sheltered from direct sunlight.