The sexually transmitted diseases are perhaps the most important infections acquired through the
urogenital tract, from the social as well as medical points of view. Three frequent infectious diseases
of this type are gonorrhea, syphilis, and chlamydial urethritis/cervicitis. All three infections are
caused by bacteria. Gonorrhea is caused by Neisseria gonorrhoeae; syphilis by Treponema pallidum, a
spirochete; and chlamydial infection by Chlamydia trachomatis. Neisseria gonorrhoeae can be grown on
special laboratory culture media, but chlamydiae are obligate intracellular parasites (once considered
viruses, in part for this reason) and require special laboratory techniques for isolation. Treponema pallidum, on the other hand, has not yet been grown in any laboratory culture
system and is cultivated only in certain animals, such as the rabbit.
The bacterial groups to which these sexually transmitted agents belong contain other
pathogenic species associated with nonsexually transmitted disease, that is, infections acquired
through other entry portals. Still other species of Neisseria and Treponema are nonpathogenic, including
some that are frequent members of the normal flora of various mucous membrane surfaces,
particularly of the respiratory tract.