Coxiella burnetii, the cause ofQ fever,has morphologic features similar to those of rick-ettsiae but differs in DNA composition and a number of other features. Phase variation of surface polysaccharide in response to environmental conditions has been observed and linked to virulence. The organism is taken into host cells by a phagocytic process that in contrast to rickettsiae does not involve expenditure of energy by the parasite. It multiplies in the phagolysosome primarily because it is adapted to growth at low pH and resists lysoso-mal enzymes. C. burnetii is much more resistant to drying and other environmental condi-tions than rickettsiae, which substantially accounts for its ability to produce infection by the respiratory route.