A syphilis sore (chancre) is classically a single, painless, firm round ulcer at the site of infection, while mpox lesions are usually multiple, often painful or itchy, and frequently develop a dimpled (umbilicated) center. Mpox more often brings fever and swollen glands first. The overlap is real, so only a swab or blood test gives the answer.
vaccine + tecovirimat
curable with penicillin; congenital cases surging
| Item | Value |
|---|---|
| Mpox | curable — vaccine + tecovirimat |
| Syphilis | curable — curable with penicillin; congenital cases surging |
The bottom-line difference
Think of it as one sore versus a crop of bumps. Primary syphilis tends to show up as a lone, painless ulcer with clean, firm edges that you might not even notice. Mpox tends to produce several lesions that look like pimples or blisters, often sore or itchy, and many sink in the middle as they mature. But the two overlap enough that you usually can't tell them apart by feel or by a phone photo, so get tested to settle it definitively.
What each one is
Mpox
Mpox is caused by the monkeypox virus, a relative of the smallpox virus. The clade II strain drove the global outbreak that began in 2022, spreading mainly through close skin-to-skin contact — including intimate and sexual contact — and affecting mostly men who have sex with men CDC, About Mpox. Most people recover fully, but it can be severe in people who are immunocompromised, such as those with advanced HIV.
Syphilis
Syphilis is a bacterial infection caused by Treponema pallidum. Unlike a virus, it's curable with the right antibiotics CDC, About Syphilis. It moves through stages — primary, secondary, latent, and late — each with a different look and different stakes, so catching it early matters.
Symptoms compared
Mpox starts with a rash that can look like pimples or blisters — often painful or itchy — on the hands, feet, face, mouth, genitals, or anus. In sexually transmitted cases, lesions frequently cluster in the genital or anal area or the mouth. Many people also get fever, swollen lymph nodes, muscle aches, exhaustion, and respiratory symptoms; some develop the rash with no warning prodrome at all CDC, Mpox symptoms.
Primary syphilis announces itself with one or more painless, firm, round sores (chancres) at the exact spot the bacteria entered — the penis, vagina, anus, rectum, lips, or mouth. The chancre lasts roughly three to six weeks and heals on its own whether or not you treat it, which fools people into thinking the problem is gone when the infection is still there. If untreated, it advances to the secondary stage: a rough red or reddish-brown rash that can appear on the palms and soles or across the body, plus mucous-membrane lesions, fever, swollen lymph nodes, sore throat, patchy hair loss, headache, weight loss, muscle aches, and fatigue.
How to tell them apart
A few discriminating features lean one way or the other, though none is proof on its own:
- Pain: a syphilis chancre is classically painless, whereas mpox lesions are often painful or itchy. A sore that hurts tilts toward mpox.
- Number: syphilis usually starts with a single sore (sometimes a few); mpox more often produces multiple lesions in a crop.
- Shape: mpox lesions commonly develop a dimpled (umbilicated) center as they mature; the syphilis chancre stays a clean, firm, round ulcer.
- Systemic warning signs: fever, swollen glands, and body aches can occur with both, but a prodrome of fever before the rash points toward mpox.
- Evolution: mpox lesions march through stages — flat spots, raised bumps, fluid-filled, pus-filled, then scab and fall off — while a chancre simply ulcerates and slowly heals.
Secondary syphilis can produce a body rash and swollen nodes that mimic a viral illness, and mpox can cause a genital ulcer that looks like a chancre. Test rather than guess.
Side-by-side comparison
| Feature | Mpox | Syphilis (primary) |
|---|---|---|
| Cause | Monkeypox virus (smallpox family) | Treponema pallidum bacterium |
| Typical lesion | Pimple- or blister-like, often umbilicated | Single firm, round ulcer (chancre) |
| Pain | Often painful or itchy | Usually painless |
| Number | Often multiple | Usually one (sometimes a few) |
| Systemic symptoms | Fever, swollen nodes, aches, fatigue, respiratory | Often none in primary stage |
| How it heals | Scabs over and falls off; recover with care | Heals on its own, but infection persists |
| Diagnosis | PCR swab of a lesion | Two blood tests (nontreponemal + treponemal) |
| Treatment | Supportive care; antiviral for severe cases | Penicillin injection — curable |
Testing
Mpox is confirmed by detecting mpox virus DNA by PCR from a swab of a suspected lesion. There's no useful blood test for the active rash, so a clinician needs to touch a sore directly. Details on the swab and what results mean are covered in our guide to mpox testing.
Syphilis diagnosis requires two serologic blood tests working together: a nontreponemal test (RPR or VDRL) and a treponemal test (TP-PA, FTA-ABS, EIA, or CIA) CDC Lab Recs, 2024. One test alone can mislead, since false positives and false negatives happen, so the pairing is the standard. In practice, testing means a urine sample, a self-collected swab, or a quick exam depending on which infection is suspected, and it's free or low-cost at health departments, Planned Parenthood, and Title X clinics. Timing matters too; antibody tests can miss a very recent infection, so review when to test after exposure before assuming a negative is final.
Treatment compared
Most people with mpox and without severe disease recover with supportive care and pain management as the lesions run their course. The antiviral tecovirimat (TPOXX) is considered for severe disease or people at high risk, such as those with advanced HIV, through CDC access; trials found it safe but it did not speed lesion healing CDC, Mpox treatment.
Syphilis is curable, and penicillin is still the only reliable cure. For primary, secondary, and early latent syphilis the standard is benzathine penicillin G (Bicillin L-A) given as a single intramuscular injection CDC P&S Syphilis Tx. The combination product Bicillin C-R (benzathine plus procaine) is not an acceptable substitute and has caused treatment-failure errors. Late or unknown-duration disease needs three weekly injections instead CDC Latent Syphilis Tx, and there's no oral drug that dependably replaces the shot.
Many people feel fever, chills, and headache within a couple of hours of that first injection. That's the Jarisch-Herxheimer reaction — dying bacteria releasing inflammatory signals, not a penicillin allergy — and it affects roughly 95% of people treated for secondary syphilis, settling within about a day StatPearls, J-H reaction. It's expected; don't let it stop the course.
Can you have more than one at once?
Yes. The same close skin-to-skin and sexual contact that spreads mpox also spreads syphilis, so co-infection is possible, and a genital ulcer can sit right next to mpox lesions. Both also share risk with HIV. Because the look overlaps and one infection doesn't rule out another, a clinician will often test broadly rather than stop at the first match. If you're pregnant or planning to be, syphilis carries serious risks to the baby — see syphilis in pregnancy for screening and treatment timing.
When to see a clinician
See a clinician promptly for any new genital, anal, or oral sore, a spreading rash, a rash on the palms or soles, or sores paired with fever and swollen glands — especially after a new or anonymous partner. Don't wait for a chancre to heal as proof you're fine; syphilis sores vanish on their own while the bacteria stay. And if you're immunocompromised and develop a pox-like rash, get seen quickly, since mpox can be more severe.