Swollen lymph nodes in the groin can be a sign of an STI, most often genital herpes (especially a first outbreak), syphilis, or recently acquired HIV. But many groin swellings come from ordinary skin or leg infections. Because these causes overlap, a test tells you which one it is.
Herpes simplex virus
Treponema pallidum
Human immunodeficiency virus
| Item | Value |
|---|---|
| Genital herpes | managed — Herpes simplex virus |
| Syphilis | curable — Treponema pallidum |
| HIV | managed — Human immunodeficiency virus |
Why STIs make groin lymph nodes swell
The lymph nodes in your groin (the inguinal nodes) drain the genitals, lower abdomen, and legs. When an infection lands in that territory, those nodes fill with immune cells and enlarge, and you feel a tender or firm lump in the crease where your thigh meets your pelvis. With an STI, the swelling is your body reacting to the bug, so it's the pattern of other symptoms — sores, rash, fever — that points to the cause.
Which STIs cause swollen lymph nodes in the groin
Genital herpes
Genital herpes is caused by two viruses, herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) CDC, About Herpes. Most people have no or very mild symptoms and never know they're infected, and the majority of HSV-2 infections go undiagnosed. When symptoms do show, the giveaway is a first outbreak: blisters that break into painful sores on or around the genitals, rectum, or mouth, taking a week or more to heal, often with flu-like symptoms — fever, body aches, and swollen glands that frequently include the groin nodes. Repeat outbreaks are shorter and milder, and some people feel a prodrome (tingling or itching) before sores appear. If you want to understand soothing and suppressive options, see alternative herpes treatments.
Syphilis
Syphilis is caused by the bacterium Treponema pallidum and is curable with the right antibiotics CDC, About Syphilis. The primary stage produces one or more painless, firm, round sores (chancres) at the site of infection — penis, vagina, anus, rectum, lips, or mouth. The chancre appears about three weeks after exposure (the incubation window runs roughly 10 to 90 days), lasts three to six weeks, and heals on its own whether or not you're treated. A painless genital sore with a nearby firm groin node is classic for syphilis. Left untreated, it moves to the secondary stage: a rough red or reddish-brown rash that can show up on the palms and soles, along with mucous-membrane lesions, fever, swollen lymph nodes, sore throat, patchy hair loss, headache, weight loss, muscle aches, and fatigue. For the full picture of how it progresses, read what is syphilis? causes, stages & risks.
HIV (early infection)
HIV is a virus that attacks the immune system CDC, About HIV. Within two to four weeks of infection, many people develop a flu-like illness called acute retroviral syndrome — fever, chills, rash, night sweats, muscle aches, sore throat, fatigue, mouth ulcers, and swollen lymph nodes, including in the groin HIV.gov, Symptoms. The swelling here tends to be more generalized, hitting the neck, armpits, and groin together rather than a single tender lump. Some people have no symptoms at all, so a test is the only way to know. After this acute phase, HIV often goes years without symptoms during clinical latency, even though the virus stays active. Starting treatment early protects your health and also lowers the chance of passing it on — see earlier hiv treatment can help prevention.
When it's NOT an STI
Plenty of groin swelling has nothing to do with sex. The same inguinal nodes drain your legs and skin, so an infected cut, an ingrown hair, athlete's foot, a boil, or cellulitis (a spreading bacterial skin infection) on the leg or genitals can all light up a groin node. These tend to be tender and come with an obvious local cause — a red, sore patch, a scratch, a pimple-like bump. A hard, fixed, painless node that lingers deserves a doctor's look regardless of any sexual history, since not every persistent lump is an infection.
How to tell them apart
You usually can't tell by looking alone. These conditions overlap too much, and several are frequently silent, so a test settles which one it is, if any. A few features steer the suspicion:
- Painful sores plus tender groin nodes and flu-like symptoms point toward a first herpes outbreak.
- A painless, firm sore with a firm nearby node points toward primary syphilis.
- Generalized swollen nodes (neck, armpits, and groin) with fever and rash a few weeks after a new exposure raises the question of acute HIV.
- A tender node next to an obvious cut, boil, or red leg suggests an ordinary skin or leg infection.
- The tender-versus-firm quality of the node is a useful clue, but it's never proof.
Side-by-side comparison
| Cause | Node feel | Tell-tale companion signs | Typical timing after exposure |
|---|---|---|---|
| Genital herpes | Tender | Painful blisters/sores, fever, body aches (first outbreak) | Not specified on current CDC pages |
| Syphilis (primary/secondary) | Firm, often painless | Painless sore; later a rash on palms/soles | Chancre ~3 weeks (range 10–90 days) |
| HIV (acute) | Often generalized | Flu-like illness, rash, sore throat, night sweats | Symptoms (if any) 2–4 weeks |
| Skin/leg infection (not an STI) | Tender | Obvious cut, boil, ingrown hair, red leg | Tracks the local injury |
How it's tested
Testing depends on what's suspected: a herpes sore can be confirmed by type-specific virologic testing of the lesion (a swab using NAAT or culture, which works best on a fresh sore) CDC, Herpes Testing; syphilis takes two blood tests — a nontreponemal test (RPR or VDRL) plus a treponemal test (TP-PA, FTA-ABS, EIA, or CIA) CDC, 2024; and HIV uses a blood or rapid test, with results conclusive only after the window period CDC, HIV Testing. In practice that means a urine sample, a self-collected swab, or a quick exam, often free or low-cost at health departments, Planned Parenthood, and Title X clinics, with results usually back in a few days. To start, get tested, and if you're counting days since a risk, check when to test after exposure.
What to do next
Because the symptoms overlap, you usually can't self-diagnose this, and a test turns a guess into an answer. If you have a sore, get the sore itself swabbed before it heals. If your only sign is a swollen node, a clinician can examine you and order the right blood tests. Each of these infections has a clear path: herpes is managed with antiviral medicine, syphilis is cured with the right antibiotics, and HIV is controlled with daily treatment that keeps you healthy. Once you know the diagnosis, the treatment is straightforward.
Red flags — when to get seen urgently
- A spreading area of red, hot, painful skin with fever — possible cellulitis needing prompt antibiotics.
- A node that's rapidly enlarging, hard, fixed in place, or that keeps growing over weeks.
- A painless genital sore — get it checked even though it doesn't hurt, because syphilis presents this way.
- Flu-like illness with rash and swollen glands a few weeks after a new sexual exposure.
- Sores so painful you can't urinate, or a high fever with a first herpes outbreak.