A skin rash linked to a sexually transmitted infection is most often caused by one of three culprits: syphilis, HIV, or mpox. A rough rash on the palms and soles strongly suggests secondary syphilis; a flu-like rash a few weeks after exposure can be acute HIV; and painful or itchy blister-like bumps point toward mpox. Testing settles which one it is.
Treponema pallidum
Human immunodeficiency virus
Monkeypox virus
| Item | Value |
|---|---|
| Syphilis | curable — Treponema pallidum |
| HIV | managed — Human immunodeficiency virus |
| Mpox | curable — Monkeypox virus |
Plenty of rashes have nothing to do with sex at all, and the STI rashes overlap enough that you usually can't name them by sight. Below is what each one actually looks like, where it shows up, how clinicians tell them apart, and when a rash means you need to be seen now.
Which STIs cause a skin rash?
Syphilis
Syphilis is caused by the bacterium Treponema pallidum and is curable with the right antibiotics CDC. It moves through stages, and the rash is the hallmark of the secondary stage. The first sign, the primary stage, isn't a rash at all. It's one or more painless, firm, round sores called chancres at the spot where the bacterium entered: the penis, vagina, anus, rectum, lips, or mouth. The chancre typically appears about three weeks after exposure (the incubation range runs roughly 10 to 90 days) and heals on its own in three to six weeks whether or not you treat it. That healing is misleading, because the infection is still there.
Weeks to months later, secondary syphilis brings the rash people search for: a rough, red or reddish-brown eruption that classically appears on the palms and soles and/or across the trunk. It usually doesn't itch. It often travels with other clues like fever, swollen lymph nodes, sore throat, patchy hair loss, headache, weight loss, muscle aches, and fatigue, plus moist sores on mucous membranes (in the mouth or genitals) that are loaded with bacteria and highly contagious. A palms-and-soles rash is unusual for most everyday skin conditions, so it's a red flag for syphilis. Learn how the stages connect in what is syphilis? causes, stages & risks.
HIV
HIV is a virus that attacks the immune system CDC. Within about two to four weeks of infection, many people develop a flu-like illness called acute retroviral syndrome, and a rash is one of its features hiv.gov. This rash tends to be a diffuse, slightly raised or flat reddish eruption on the trunk, showing up alongside fever, chills, night sweats, muscle aches, sore throat, fatigue, swollen lymph nodes, and mouth ulcers. Because it looks and feels like the flu or mono, it's easily dismissed.
After the acute phase, HIV enters clinical latency, often years with no symptoms at all while the virus stays active, before the most advanced stage, AIDS, when the immune system is depleted enough to allow opportunistic infections. Some people get no acute symptoms whatsoever, so don't wait on a rash. Starting treatment early protects both your health and your partners; here's how earlier hiv treatment can help prevention.
Mpox
Mpox is caused by the monkeypox virus, a relative of smallpox CDC. The clade II strain behind the outbreak that began in 2022 spreads mainly through close skin-to-skin contact, including intimate and sexual contact, and has affected mostly men who have sex with men. The rash is its defining symptom and looks distinctly different from the others: firm bumps that resemble pimples or blisters, often painful or itchy, which evolve over days and then scab over CDC.
Mpox lesions favor the hands, feet, face, mouth, genitals, and anus, and in sexually transmitted cases they frequently start in the genital or anal area or the mouth, sometimes as just a few bumps. Fever, swollen lymph nodes, muscle aches, exhaustion, and respiratory symptoms may come first, but some people get the rash with no warning. The incubation period is 3 to 17 days, most people fall ill within 21 days of contact, and the illness usually runs 2 to 4 weeks. Most recover fully, though it can be severe in people who are immunocompromised. More on transmission and the vaccine in our guide to mpox.
When it's not an STI
Not every rash is an infection, let alone a sexually transmitted one. A new rash can just as easily come from eczema (a chronic, itchy, dry-skin condition that flares with irritants), an allergic reaction (to a new soap, detergent, latex, or medication), or an ordinary viral rash that follows a common cold or other infection. Heat, friction, and contact dermatitis cause genital rashes all the time. The most useful distinguishing clue is location: a rash that includes the palms and soles points strongly toward secondary syphilis, since most everyday skin conditions spare those areas.
How to tell them apart
You generally can't. These rashes overlap too much to separate by sight alone, and several of these infections are frequently silent or produce vague symptoms. A few patterns help you narrow the odds before testing:
- A non-itchy, rough rash on the palms and soles, especially with swollen glands and patchy hair loss, fits secondary syphilis.
- A diffuse rash on the trunk arriving with fever, sore throat, and night sweats a couple of weeks after a risky exposure fits acute HIV.
- Painful or itchy blister-like bumps that scab over, often starting on the genitals, anus, or mouth, fit mpox.
- An itchy, dry, recurring rash with no fever and no recent exposure more often points to eczema or an allergy.
Even with those patterns, a test tells you which infection, if any, you have. Overlapping presentations make self-diagnosis fail here, and a quick test turns a worried guess into a real answer.
Side-by-side comparison
| Cause | What the rash looks like | Where it shows up | Timing after exposure | Other clues |
|---|---|---|---|---|
| Secondary syphilis | Rough, red/reddish-brown; usually not itchy | Palms, soles, and/or trunk | Weeks to months (chancre first, ~3 weeks) | Swollen nodes, hair loss, fever, sore throat, fatigue |
| Acute HIV | Diffuse, flat-to-raised, reddish | Trunk | 2–4 weeks | Fever, night sweats, sore throat, mouth ulcers |
| Mpox | Pimple- or blister-like; painful or itchy; scabs over | Genitals, anus, mouth, hands, feet, face | 3–17 days (illness lasts 2–4 weeks) | Fever, swollen nodes, muscle aches, exhaustion |
| Eczema / allergy / viral | Itchy, dry, or hive-like; varies | Anywhere; often spares palms/soles | Tied to an irritant or recent illness, not sex | No fever from STI; often recurring |
How it's tested
Each of these has its own confirmatory test: syphilis takes two blood tests, a nontreponemal test (RPR or VDRL) plus a treponemal test (TP-PA, FTA-ABS, EIA, or CIA) CDC lab guidance, 2024; HIV uses a blood test whose accuracy depends on the window since exposure CDC; and mpox is confirmed by PCR on a swab of a lesion. In practice testing is a urine sample, a self-collected or clinician swab, or a quick exam depending on what's suspected, and it's free or low-cost at health departments, Planned Parenthood, and Title X clinics with results usually back in a few days. See your options and what to expect on our get tested page, and check when to test after exposure so a negative actually means something.
What to do next
Don't try to wait the rash out or guess from photos online. Get tested for the relevant infections based on your exposure and symptoms, and bring up anything that fits the patterns above so the clinician orders the right panel. Syphilis is curable with antibiotics, mpox has antiviral options for people who need them CDC, and HIV is highly manageable when caught early. If a test comes back positive, treatment is straightforward. Start with our overview of next steps and link out from there.
Red flags — when to get seen urgently
Some situations shouldn't wait for a routine appointment. Seek care promptly if you have:
- A rash on the palms and soles, which strongly suggests secondary syphilis and needs treatment.
- Painful blister-like lesions on the genitals, anus, or mouth, especially with fever. Get evaluated and avoid skin-to-skin contact until you know what it is.
- A flu-like illness with a rash 2–4 weeks after a high-risk exposure, which can be acute HIV, the most contagious phase.
- Rapidly spreading lesions, severe pain, trouble swallowing, eye involvement, or any rash in someone whose immune system is compromised.