There are more than 25 known sexually transmitted diseases, which can be caused by a variety of agents. Bacterial diseases include syphilis, gonorrhea, chancroid, granuloma inguinale, and lymphogranuloma venereum (caused by chlamydia) and chlamydia itself. Viral pathogens including herpes simplex virus, human papilloma virus, human immunodeficincy virus, hepatitis B and C, molluscum contagiosum and human T-lymphocyte virus (HTLV). Trichomonal vaginitis is caused by a protozoan while pediculosis pubis is caused by a louse.

A bacterial agent, Treponema pallidum, causes syphilis. This disease affects many organs, including the genitals, skin and mucus membranes, the central nervous system and heart, among other organs. If left untreated, syphilis goes through four stages in the human body. The primary phase is characterized by a chancer, a painless ulcer which appears 10–90 days after incubation (average of 21 days). These ulcers may appear on the mucus membranes (vagina, mouth, and cervix) or on the breast, vulva or anus. Dark-field microscopy will demonstrate the spirochete (T. pallidum) if obtained from serum oozing off the chancer. Primary syphilis usually heals in 3–6 weeks. If left untreated, syphilis can lead to serious complications or death, but with early diagnosis and treatment the disease can be successfully treated.

A single chancer is usually typical but there may be multiple sores. Enlarged lymph nodes in the groin may be associated with the chancer. Primary syphilis typically disappears without any treatment, but the underlying disease remains, and may reappear at the secondary or tertiary stage.

The signs and symptoms of secondary syphilis may begin 2–10 weeks after the chancer appears and may include rash marked by red and reddish-brown, penny-size sores over any area of the body, including palms of the hands and soles of the feet, fever, fatigue and feeling of discomfort, soreness and aching, condyloma latum (painful lesion in the anogenital area or a characteristic rash). Dark-field microscopy or biopsy is diagnostic at this stage. In some people a period called latent syphilis, in which no symptoms are present, may follow secondary syphilis. Signs or symptoms may never return or disease may progress to the tertiary stage. The latent stage has two phases: the early latent phase occurs within 1 year after acquiring the infection and the late latent phase if it is more than a year.

Without treatment, the disease may progress to the tertiary stage. During this stage, syphilis bacteria may spread, leading to serious internal organ damage and death years after the original infection. The main organs that may be affected include cardiac, neurologic, ophthalmic, and auditory systems.

The serologic tests for syphilis may be classified into two groups. Nonspecific treponemal antibodies include the Venereal Disease Reseach Laboratory test (VDRL) and rapid plasma reagin (RPR). Treponemal tests, which detect specific treponemal antibodies, include Treponema pallidum hemagglutination (MHA-TP), the fluorescent treponemal antibody absorb test (FT-ABS) and most of the new treponemal enzyme immunoassay tests.

An important principle of syphilis serology is the detection of treponemal antibody by screening tests, followed by confirmation of a reactive screening test result by further testing. The confirmatory test or tests should ideally have a lower sensitivity and greater specificity than the screening test and use independent methodology.

The serologic markers usually become positive 4–5 weeks after the initial infection, or 1–2 weeks after the appearance of a chancer. A titer greater than 1:32 is diagnostic, while a titer less than 1:32 should be repeated. A fourfold change in the titer is usually considered clinically significant (e.g. an increase in a titer from 1:4 to 1:16 in a previously treated person might signify reinfection). Most patients’ serologic tests will remain reactive once they have become positive. Of those patients treated in the primary stage, only 15–25% might revert to nonreactive serology, while those treated at other stages usually remain positive.

Treatment of syphilis involves a single dose of benzathine penicillin G 2.4 million units IM for patients diagnosed with primary, secondary or early latent stage. Those with a penicillin allergy can be treated with doxycycline 100 mg twice a day for 14 days, or tetracycline 500 mg four times a day for 14 days, or erythromycin 500 mg orally four times a day for 14 days. Tertiary syphilis or the late latent phase can be treated with benzathine penicillin G 2.4 million units IM once a week for 3 weeks (total of 7.2 million units), or doxycycline, tetracycline, and erythromycin taken in the same manner as above for 1 month.