Chlamydiaceae is a family of obligate intracellular parasites closely related to gram-negative bacteria. They include two important genera of human pathogens—Chlamydia, which includes the species of Chlamydia trachomatis, and Chlamydophila, which includes the species Chlamydophila psittaci (formerly known as Chlamydia psittaci) and Chlamydophila pneumoniae (formerly known as Chlamydia pneumoniae). The differentiation of genera is based on differences in intracellular inclusions, sulfonamide susceptibility, antigenic composition, and disease production. C trachomatis causes many different human infections involving the eye (trachoma, inclusion conjunctivitis), the genital tract (lymphogranuloma venereum, nongonococcal urethritis, cervicitis, salpingitis), or the respiratory tract in infants (pneumonitis). C psittaci causes psittacosis and C pneumoniae is a cause of respiratory tract infections.