Rare STDs You've Never Heard Of (But Should Know About)

Beyond the well-known names — chlamydia, gonorrhea, herpes, HIV — there are several STDs that most people have never heard of but that are becoming more common, more difficult to treat, or more likely to be misdiagnosed. Knowing they exist is the first step in recognising them and seeking the right test.

• Mycoplasma genitalium is more common than gonorrhea but absent from most standard STI panels

• Lymphogranuloma venereum (LGV) is frequently misdiagnosed as inflammatory bowel disease

• Donovanosis is rare in Western countries but cases are increasing with international travel

• Trichomoniasis affects an estimated 3.7 million Americans but 70% have no symptoms

• Molluscum contagiosum can persist for months and is spread through skin contact, not just sex

Mycoplasma Genitalium: The Antibiotic-Resistant Hidden STI

Mycoplasma genitalium (MG) is a bacterial STI that is more prevalent than gonorrhea in many populations, yet it is not included in routine STI screening at most clinics. First identified in 1981, it was only recently recognised as a significant STI in clinical guidelines.

MG causes urethritis in men (painful urination, clear discharge) and cervicitis in women (abnormal discharge, pain during sex, spotting between periods). Up to 70% of infections produce no symptoms at all. Left untreated in women, it can progress to Pelvic Inflammatory Disease and infertility. Treatment is complicated by high rates of antibiotic resistance — macrolide resistance (to azithromycin) has reached 50–80% in some regions. A specific NAAT test is required to detect it, and you must ask for it by name.

Lymphogranuloma Venereum (LGV): The Misdiagnosed Chlamydia Variant

LGV is caused by specific invasive strains of Chlamydia trachomatis (serovars L1, L2, L3) that behave very differently from the chlamydia strains that cause standard genital infections. While common chlamydia infects the mucous membranes of the genitals and cervix, LGV strains invade the lymphatic system and can cause severe damage if not caught.

LGV progresses through three stages. The first produces a small, painless sore or blister at the site of infection — often unnoticed. The second causes painful swelling of the lymph nodes in the groin (buboes), accompanied by fever and fatigue. The third, if left untreated, causes severe rectal inflammation, chronic pain, fistulas, and rectal bleeding that can be permanently disfiguring. This late-stage presentation is frequently misdiagnosed as Crohn’s disease or inflammatory bowel disease, delaying the correct treatment — a prolonged course of doxycycline.

LGV cases have been rising in Europe and North America since 2003, predominantly among men who have sex with men (MSM), though it can infect anyone. Standard chlamydia tests detect LGV, but only specialist labs can confirm which specific strains are present. If you have symptoms of rectal LGV and standard chlamydia treatment is not working, ask specifically about LGV strain typing.

Trichomoniasis: The Most Common Curable STD You’ve Probably Never Been Tested For

Trichomoniasis is caused by a parasite — Trichomonas vaginalis — and is estimated to be the most common non-viral STD globally. The CDC estimates that approximately 3.7 million Americans have it at any given time. Despite this prevalence, it is not included in most standard STI panels, meaning many people carry it without ever being tested.

Around 70% of infected individuals experience no symptoms. When symptoms do appear, women may notice frothy, yellow-green vaginal discharge with a strong odour, genital itching, and burning during urination. Men rarely have symptoms but can still transmit the infection. Trichomoniasis is fully curable with a single dose of metronidazole or tinidazole, but it increases susceptibility to HIV and has been associated with pregnancy complications including preterm birth and low birth weight. Ask your clinician specifically to include trichomoniasis in your testing panel — it requires a separate test.

Donovanosis (Granuloma Inguinale): The Rarely Seen But Serious Ulcerative STD

Donovanosis is a bacterial STI caused by Klebsiella granulomatis. It is rare in Western countries but endemic in parts of India, Papua New Guinea, South Africa, and South America. Cases in the UK, US, and Europe do occur — typically linked to travel to endemic regions or sexual contact with someone from those areas.

It presents as painless, progressive ulcers on the genitals that slowly enlarge and bleed easily. Unlike herpes, the sores are not painful. Without treatment, the lesions continue to expand, destroying genital tissue over time — which has led to its informal description as a “flesh-eating” STD, though this framing overstates its typical presentation. It can also spread to the groin, abdomen, and rarely to distant organs. Donovanosis is treatable with antibiotics (azithromycin, doxycycline, or ciprofloxacin) for a minimum of three weeks, but early diagnosis is essential. Diagnosis requires a tissue smear examined for the characteristic Donovan bodies — it will not be caught on standard STI panels.

Molluscum Contagiosum: The Persistent Viral Skin Infection

Molluscum contagiosum is caused by a poxvirus and spreads through direct skin-to-skin contact. In adults, it is frequently transmitted sexually, though it can also spread through shared towels, clothing, and gym equipment. It causes small, round, flesh-coloured or pearly bumps with a characteristic central dimple, typically appearing on the genitals, inner thighs, lower abdomen, or buttocks in sexually transmitted cases.

The bumps are usually painless but can itch. They can persist for 6–18 months and may spread to other areas of the body through scratching. In most healthy adults, the immune system eventually clears the infection without treatment. Faster resolution can be achieved through cryotherapy (freezing), curettage (scraping), or topical treatments. Molluscum does not appear on standard STI tests — it is diagnosed by clinical examination. Importantly, it remains transmissible throughout the active infection period, including when no symptoms are obvious.

When to Ask About These Infections

None of the five infections above are routinely included in standard STI panels. You need to ask for them specifically, or at least describe your symptoms in enough detail that a clinician considers them. Ask about MG testing if you have persistent urethritis or cervicitis that does not respond to standard treatment. Ask about LGV if you have rectal symptoms and are in a higher-risk group. Ask about trichomoniasis as part of any comprehensive STI screen. Mention recent travel to endemic regions if you have unusual genital ulcers. And have any unexplained skin bumps in the genital area examined clinically.

Tips for Comprehensive STI Testing

• Tell your clinician about all symptoms — including rectal, oral, and skin symptoms, not just genital ones.

• Mention recent travel — particularly to South Asia, Sub-Saharan Africa, or Papua New Guinea if you have genital ulcers.

• Ask what is and is not included in your standard panel — most do not cover MG, trichomoniasis, or LGV strains.

• Request a comprehensive panel if you have multiple partners or have had unprotected sex with someone whose status you do not know.

• Follow up if treatment is not working — persistence of symptoms after standard treatment is a signal that the right pathogen may not yet have been identified.

Frequently Asked Questions

Why are these STDs not on standard testing panels?

Standard panels cover the most common and most public-health-significant infections that have well-established, widely available tests. Some rarer infections require specialist laboratory techniques that are not available in all settings. Others, like MG, were only recently recognised as clinically significant. Clinical guidelines are updating, and coverage in standard panels is gradually expanding.

Can I have one of these infections alongside a common STD?

Yes. Co-infections are common. Having one STD generally increases susceptibility to others by causing inflammation that disrupts the normal protective barriers of the genital tract. Testing for multiple infections simultaneously is always reasonable if you have been at risk.

Are these infections curable?

Trichomoniasis and donovanosis are fully curable with antibiotics. LGV is curable with a prolonged course of doxycycline. MG is treatable but increasingly difficult due to antibiotic resistance, and a test of cure is required. Molluscum contagiosum clears spontaneously in most healthy adults, though it can take many months.

Is trichomoniasis dangerous?

In healthy non-pregnant adults, trichomoniasis is uncomfortable but not life-threatening. However, it increases susceptibility to HIV, and in pregnancy it is associated with preterm birth, low birth weight, and premature rupture of membranes. It should always be treated when detected.

Can men get LGV?

Yes. LGV affects anyone regardless of gender or sexual orientation. It is currently more prevalent among MSM in Western countries due to specific transmission networks, but it infects heterosexual men and women too, particularly in regions where it is endemic.

Get Tested Today

If you have symptoms that are not explained by the usual STI suspects, or if you simply want comprehensive coverage, ask your sexual health provider to go beyond the standard panel. Early identification of these less common infections prevents complications and transmission to partners.

Related reading: Mycoplasma Genitalium: The Hidden STI · What Symptoms Could Indicate an STD? · The Rise of Antibiotic-Resistant STDs · Can You Have an STD With No Symptoms?