Chlamydiaceae is a family of Gram-negative intracellular bacteria comprising genera: Chlamydia and Chlamydophila. The Chlamydiaceae cell wall is distinguished by a three-layer outer membrane and a thin inner membrane. Lipopolysaccharide complexes are embeded in the outer membrane, but they have lower endotoxic activity than those of gram negative enterobacteria. This causes a weaker immune response, which promotes asymptomatic infection. The most important epidemiologically and clinically Chlamydiaceae species are Chalmydophila pneumoniae (formerly Chlamydia pneumoniae) which causes respiratory tract infections, Chlamydia trachomatis which causes sexually transmitted infections and Chlamydophila psittaci (formerly Chlamydia psitacci) which causes psittacosis.