acute, self-limited, systemic disease caused by the rickettsia Coxiella burnetii. Q fever spreads rapidly in cows, sheep, and goats, and in humans it tends to occur in localized outbreaks. The clinical symptoms are those of fever, chills, severe headache, and pneumonia. The disease is usually mild, and complications are rare. Treatment with tetracycline or chloramphenicol shortens the duration of illness.

Q fever was first recognized in 1935 in Queensland, Australia, by Edward Holbrook Derrick. According to Derrick, Q stands for query, an appellation applied because of the many unanswered questions posed by the new disease at the time of its first description. The disease was originally encountered among abattoir workers, cattle ranchers, and dairy farmers in Australia and later among sheep ranchers, and it was first thought to be restricted to that continent. Several outbreaks of what was ultimately shown to be Q fever, however, occurred among Axis and Allied troops in the eastern Mediterranean theatre of World War II during the winter of 1944–45. These were the first naturally occurring outbreaks of Q fever recognized outside Australia; the disease thereafter was reported from various parts of the world.

While many species of ticks (tick) in various parts of the world have been found to be naturally infected, the role of these arthropods in the dissemination and maintenance of the rickettsiae is unclear. It seems likely that some small mammal, perhaps a rodent, serves as a reservoir of the rickettsiae and that ticks keep the infection alive in nature by spreading the rickettsiae from animal to animal within the host species. Humans and their domestic livestock are not necessary to the survival of the rickettsiae in nature and are infected only accidentally.

Tick-transmitted Q fever appears to be rare in humans and, in some parts of the world at least, in domestic livestock as well. Because the rickettsiae are found in cows' and goats' milk, the ingestion of infected dairy products may play a role in the infection of humans and livestock. The common mode of infection, however, appears to be inhalation of infected material. The infected animal sheds the rickettsiae through its milk, excreta, and, most importantly, through the placenta and birth fluids. Contamination of the environment leads to airborne dissemination of the rickettsiae and thus to infection of persons in close contact with livestock, contaminated clothing, and other infected sources.

The incubation period of the disease is from two to four weeks, averaging about 18 to 21 days. The onset may be gradual but generally is sudden, and the disease is ushered in by fever, chills or chilly sensations, headache, muscle aches, loss of appetite, disorientation, and profuse sweating. Symptoms in the upper respiratory tract may be present but generally are infrequent and minimal; and pneumonia, even when relatively extensive, may be detectable only by X-ray examination. Although Q fever is, on the whole, a mild disease, it can sometimes result in severe and protracted illness. The outlook for recovery is excellent; the mortality rate is believed to be less than 1 percent. The disease is amenable to therapy with wide-spectrum antibiotics, which are highly effective. Q fever seems to be in large part an infection associated with particular occupations, and vaccines prepared from killed C. burnetii can be used to protect persons whose work makes them likely to be exposed to infection.