Sexually Transmitted Infections (STIs): Complete List, Types, and Causative Agents

Sexually transmitted infections are caused by bacteria, viruses, fungi, and parasites, and one of the most useful ways to learn them is not as a flat list of names but as a small number of groups: grouped by the kind of organism that causes them, and grouped by the syndrome they produce. This page gives the complete list of major STIs with their causative agents, organizes them both ways, and links each organism to its detailed article. It also works through the two things a list alone cannot teach: how to tell apart the organisms that cause the same syndrome (for example, the three causes of genital ulcer), and which laboratory test is used for which STI.

Multiple pathogens can cause the same clinical picture. A vaginitis may be due to Trichomonas vaginalis, Candida albicans, or bacterial vaginosis; a genital ulcer may be due to Haemophilus ducreyi, Treponema pallidum, or herpes simplex virus. This overlap is exactly why the syndrome-to-organism reasoning below matters.

The complete list of STIs by causative agent

The table lists the major sexually transmitted infections and the organism responsible for each. The sections that follow group these by pathogen type and link each organism to its full article.

STIs grouped by pathogen type

Bacterial STIs

Neisseria gonorrhoeae causes gonorrhea (urethritis, cervicitis, and, untreated, pelvic inflammatory disease). Signature clue: Gram-negative intracellular diplococci inside neutrophils in a urethral smear.

Chlamydia trachomatis is an obligate intracellular bacterium. Serovars D–K cause urethritis and cervicitis (the commonest bacterial STI in many countries); serovars L1–L3 cause lymphogranuloma venereum. It cannot be grown on ordinary media, so diagnosis is by nucleic acid amplification.

Treponema pallidum, a spirochete, causes syphilis. Its primary lesion is the painless hard chancre. It cannot be cultured; diagnosis relies on dark-field microscopy of the lesion and on serology.

Haemophilus ducreyi causes chancroid, the painful soft chancre with tender inguinal buboes, the counterpart to the painless syphilitic ulcer.

Gardnerella vaginalis is the marker organism of bacterial vaginosis, a polymicrobial overgrowth (with anaerobes such as Prevotella, Mobiluncus, and Bacteroides) that replaces the normal lactobacilli. Signature clues: clue cells on a wet mount and a positive whiff (amine) test.

Genital mycoplasmas (Mycoplasma genitalium, Ureaplasma urealyticum) are cell-wall-deficient bacteria and important causes of nongonococcal urethritis. M. genitalium is now recognized as a significant cause of urethritis in men and cervicitis in women. For what a cell-wall-deficient organism is, see cell-wall-deficient bacteria.

Klebsiella granulomatis causes granuloma inguinale (donovanosis), a chronic beefy-red genital ulcer, diagnosed by Donovan bodies in tissue smears.

Viral STIs

Herpes simplex virus (HSV), mainly HSV-2, causes genital herpes: recurrent, painful, grouped vesicles that ulcerate. It also causes neonatal herpes. Signature clue: multinucleated giant cells on a Tzanck smear (and confirmation by PCR).

Human immunodeficiency virus (HIV) causes progressive immune depletion and AIDS. The presence of other STIs, especially ulcerative ones, increases the risk of HIV transmission. Diagnosis uses antibody/antigen immunoassays, confirmatory testing, and viral load and CD4 monitoring.

Human papillomavirus (HPV) causes genital warts (condylomata acuminata) and, with high-risk types, cervical and other anogenital cancers. It is among the most common STIs worldwide.

Molluscum contagiosum virus, a poxvirus, causes small pearly papules with a central dimple (umbilication).

Hepatitis B virus is transmissible sexually and causes acute and chronic hepatitis; see hepatitis B virus.

Fungal STIs

Candida albicans causes vulvovaginal candidiasis (thick discharge, itch), especially with diabetes, antibiotic use, or immunosuppression. Signature clue: budding yeasts and pseudohyphae on a KOH preparation.

Parasitic STIs

Trichomonas vaginalis, a flagellated protozoan, causes trichomoniasis (frothy discharge, often a "strawberry cervix"). Signature clue: motile, jerky trophozoites on a saline wet mount.

Ectoparasitic STIs

Phthirus pubis (pubic lice, "crabs") and Sarcoptes scabiei (scabies) spread by close bodily contact including sex, causing genital itch; diagnosis is by seeing the parasite or its eggs.

Telling apart the organisms that cause the same syndrome

A single clinical syndrome usually has a short list of possible organisms, and distinguishing them is the core of STI microbiology. These are the groupings worth knowing.

Genital ulcer

The three classic causes, separated first by pain:

The one-line memory: soft chancre is soft and sore; hard chancre is hard and painless; herpes is painful and starts as blisters. Granuloma inguinale (K. granulomatis) and lymphogranuloma venereum (C. trachomatis L serovars) are less common ulcer/node causes.

Urethritis and cervicitis

Gonococcal versus nongonococcal is the first split. Neisseria gonorrhoeae causes gonococcal urethritis; Chlamydia trachomatis, Mycoplasma genitalium, and Ureaplasma urealyticum cause nongonococcal urethritis. The two often coexist, which is why testing covers both.

Vaginitis and vaginosis

Three causes, distinguished at the bench:

Growths and warts

Human papillomavirus causes cauliflower-like genital warts; molluscum contagiosum virus causes small umbilicated papules.

How STIs are diagnosed

STI microbiology relies on matching the right specimen and technique to the suspected organism. This overview links each method to its full article.

• Direct microscopy of stained smears. A Gram stain of urethral discharge showing intracellular Gram-negative diplococci is presumptive for gonorrhea. For the method, see Gram staining.
• Dark-field microscopy: used to see Treponema pallidum directly in fluid from a syphilitic chancre, because it cannot be cultured or stained conventionally.
• Saline wet mount: a drop of discharge under the microscope shows the motile trophozoites of Trichomonas vaginalis, and clue cells in bacterial vaginosis.
• KOH preparation: dissolves host cells to reveal the yeasts and pseudohyphae of Candida albicans; the KOH "whiff" also releases the amine odor of bacterial vaginosis.
• Tzanck smear: shows multinucleated giant cells in herpes lesions (a rapid but non-specific clue; PCR confirms).
• Culture: Neisseria gonorrhoeae grows on selective enriched media; H. ducreyi is fastidious and grows poorly. T. pallidum, C. trachomatis, and the viruses are not diagnosed by routine culture.
• Nucleic acid amplification (NAAT/PCR): the modern standard for Chlamydia trachomatis, Neisseria gonorrhoeae, Mycoplasma genitalium, HSV, and often T. vaginalis, because it is far more sensitive than microscopy or culture.
• Serology and immunoassays: the basis of syphilis testing (non-treponemal VDRL/RPR and treponemal TPHA/FTA-ABS) and HIV diagnosis (antigen/antibody immunoassays, with viral load and CD4 for monitoring).

For how genital and STI specimens are collected and transported, see genital and STI specimen collection.

Key exam facts

Where students get confused

• Genital ulcer is about pain first. Painful points to chancroid or herpes; painless points to syphilis. Pain is the fastest discriminator before any test.
• "Chancre" and "chancroid" are opposite diseases. The chancre is the painless syphilitic ulcer; chancroid is the painful H. ducreyi ulcer. The near-identical spelling hides opposite organisms and opposite pain.
• Several STIs cannot be diagnosed by culture. Treponema pallidum, Chlamydia trachomatis, and the viruses are not recovered on routine media; each needs its own approach (dark-field/serology, NAAT, or antigen/antibody testing). Reaching for culture by default will miss them.
• One syndrome, several organisms. A vaginitis or a urethritis is not one diagnosis. Testing has to cover the group (for urethritis, both gonococcal and nongonococcal), because co-infection is common.
• Bacterial vaginosis is not a classic infection by one pathogen. It is a shift in the vaginal flora, a loss of lactobacilli with overgrowth of Gardnerella and anaerobes, which is why it is diagnosed by clue cells and odor rather than by isolating a single organism.

References

1. Tille PM. Bailey and Scott's Diagnostic Microbiology. 15th ed. St. Louis: Elsevier; 2022.
2. Procop GW, Church DL, Hall GS, et al. Koneman's Color Atlas and Textbook of Diagnostic Microbiology. 7th ed. Philadelphia: Wolters Kluwer; 2017.
3. Riedel S, Hobden JA, Miller S, et al. Jawetz, Melnick & Adelberg's Medical Microbiology. 28th ed. New York: McGraw Hill; 2019.
4. Workowski KA, Bachmann LH, Chan PA, et al. Sexually Transmitted Infections Treatment Guidelines, 2021. MMWR Recomm Rep. 2021;70(4):1-187. https://doi.org/10.15585/mmwr.rr7004a1