Which bacterium causes Q fever?

Coxiella burnetii is the bacterium that causes Q fever.

C. burnetii is a tiny Gram-negative, obligate intracellular bacterium: it normally multiplies inside host cells rather than freely in ordinary laboratory culture. Its main animal reservoirs include cattle, sheep, and goats. People most often become infected by inhaling contaminated aerosols released around animal births, manure, slaughter, or contaminated materials.

The disease was first recognized in Australia in the 1930s, when investigators initially called it “query fever” because its cause was unknown. The name Coxiella honors researchers Cornelius B. Philip and Herald Rea Cox, while “Q fever” preserves the original mystery-related name. The bacterium was once grouped with rickettsiae, but modern classification places it among the Gammaproteobacteria, related more closely to organisms such as Legionella.

Brucella abortus can cause brucellosis, Francisella tularensis causes tularemia, and Rickettsia rickettsii causes Rocky Mountain spotted fever. They are important pathogens, but none is the canonical cause of Q fever.