STI bumps look different per infection. Herpes gives blisters in a cluster, HPV gives soft warts in bunches, and syphilis classically gives one firm sore that does not hurt. Shape, feel and course together tell you the most.
Looking is not a test, though. In the first half of 2025 the Dutch RIVM recorded 7,290 gonorrhoea diagnoses and 5,187 chlamydia diagnoses, against 944 for syphilis (RIVM). Those first two often produce no bump at all.
What strikes me about the questions we get on this: almost everyone starts at "which STI is this". The first question is more often whether it is an STI.
These are the nine pictures we see come past most often.
- Blisters in a cluster, tender: genital herpes
- Soft warts in bunches, painless: HPV
- One firm sore without pain: syphilis, first stage
- Blotchy rash on palms and soles: syphilis, second stage
- Pearly bumps with a central dimple: molluscum contagiosum
- Itching with fine burrows, worse at night: scabies
- Itching in the pubic hair with blue-grey spots: pubic lice
- Red, shiny patches on the glans: balanitis or candida
- A neat row of small white bumps: pearly papules or sebaceous glands, not an STI
What do STI bumps look like?
STI bumps differ mainly in shape, number and feel. Blisters in a cluster point more towards herpes. Soft warts in bunches fit HPV. One firm, painless sore fits syphilis. Itching without a clear bump points more often to scabies, pubic lice or a fungus.
Three questions usually get you furthest. Is it one, or are there several? Does it hurt, or does it itch? And how fast did it appear?
That last question gets skipped most often.
1. Blisters in a cluster: genital herpes
A genital herpes outbreak consists of small blisters, single or clustered. They sit on the genitals, the perineum, the buttocks or the upper thighs. The blisters usually break open and then crust over.
Many people first feel tingling or a burning spot before anything is visible. Herpes simplex affects more than 400 million people worldwide (Groves, 2016).
HSV-1 and HSV-2 cannot be told apart by eye, not even by a doctor (Groves, 2016). During the blister phase, a PCR from the blister is the usual way to confirm it. Once the outbreak has passed, there is little left to sample.
There is more on the course in genital herpes: first outbreak and transmission.
2. Soft warts in bunches: HPV
Genital warts are soft, skin-coloured bumps that often grow in groups. They usually do not hurt and rarely itch. The picture people recognise is a cauliflower-like bunch that slowly gets bigger over weeks.
They sit on the penis, the labia, in the groin or around the anus. A standard STI panel does not find them, because it carries no HPV test for men.
So this is one where looking beats a blood draw. What genital warts look like exactly is covered in a separate piece.
3. One firm sore without pain: syphilis, first stage
The first stage of syphilis classically gives one sore at the site of contact. It feels firm at the edges, it usually does not hurt, and it heals on its own. That last part is exactly the trap.
The sore also disappears without treatment while the infection can persist (Peeling and colleagues, 2017). People then conclude it is over.
Say two people spot a 6 millimetre bump on a Sunday evening that was not there on Friday. For one it hurts and holds fluid, for the other it feels firm and painless. Same bump, same day, two different stories.
The painful, fluid-filled one fits herpes better. The firm, painless one fits this sore better, especially if it clears on its own after ten days.
The RIVM recorded 944 syphilis diagnoses in the first half of 2025. Syphilis can be detected in blood, through the syphilis test. The signs per stage are set out in syphilis symptoms.
4. Blotchy rash on palms and soles: syphilis, second stage
Weeks to months after that sore, syphilis can cause a rash across the trunk, the palms and the soles. The spots usually do not itch. A rash that sits on palms and soles is unusual, which is what makes it stand out.
This stage is regularly mistaken for something else, for instance a reaction to medication (Peeling and colleagues, 2017).
I list it separately, because people searching for "STI bumps" rarely think about their hands.
5. Pearly bumps with a central dimple: molluscum contagiosum
Molluscum contagiosum lesions are firm, dome-shaped bumps with a sheen. The dimple in the middle is the giveaway. They are usually 2 to 5 millimetres across and cause no pain.
In children they appear all over the body. In adults they are less common and are often sexually transmitted (Chen and colleagues, 2013).
In people with a healthy immune system they regularly clear on their own (Chen and colleagues, 2013). No lab test exists for them. A doctor identifies them by looking.
6. Itching with fine burrows: scabies
Scabies causes intense itching that often gets worse at night. On the skin you can sometimes see fine, winding burrows a few millimetres long. They favour the spaces between the fingers, the wrists, around the navel and around the genitals.
The itch comes from a reaction to the mite, and can carry on for weeks after successful treatment (Chandler and Fuller, 2019). That is confusing, because it feels as though the treatment failed.
Here too there is no blood test. A doctor looks, sometimes with a magnifier or a dermatoscope.
7. Itching in the pubic hair with blue-grey spots: pubic lice
Pubic lice cause itching in the pubic hair. You can sometimes see the lice themselves, or their eggs glued to the hairs close to the skin. Blue-grey spots can appear at the bite sites.
Pubic lice do not always stay down there. They can also reach armpit hair, chest hair, eyebrows and eyelashes (Coates and colleagues, 2020).
No STI panel includes pubic lice, and it does not need to. This is one you can often spot yourself with a mirror and good light.
8. Red, shiny patches on the glans: balanitis or candida
A red, shiny or blotchy glans with itching or a burning feeling is called balanitis. Sometimes there is a white coating, which can point to candida. It reads as irritation rather than a genuine bump.
Often it is not an STI. Soap, a tight foreskin, moisture or a fungus can all produce the same picture.
It still cannot be ruled out, because gonorrhoea and chlamydia can irritate the same spot in men. There is more in balanitis and when it is an STI.
9. A neat row of small white bumps: pearly papules or sebaceous glands
This may be the most important of the nine, because this is not an STI. Pearly penile papules are bumps of 1 to 2 millimetres, sitting in one or two tidy rows around the rim of the glans. It is normal anatomy and not transmissible.
Sebaceous glands, also called Fordyce spots, look like small yellowish-white dots. They sit on the shaft, on the labia or on the lips. Also normal.
And then the ingrown hair: a red, tender bump shortly after shaving, sometimes with a hair in the middle of it.
I put these on the list deliberately. A good share of the questions we get on this turn out to be about something that was always there, and that only got noticed after a nervous week.
Which bump belongs to which STI?
Below you see per picture what it usually is, how it feels and what identifies it. Pay attention to the last column. For four of the nine there is no lab test and a doctor looks instead.
| What you see | Fits | Pain or itch | What identifies it |
|---|---|---|---|
| Blisters in a cluster | Genital herpes | Tender, burning | PCR from an open blister |
| Soft warts in bunches | HPV | Usually neither | Doctor looks at the warts |
| One firm, painless sore | Syphilis, first stage | No pain | Blood test |
| Rash on palms and soles | Syphilis, second stage | Usually no itch | Blood test |
| Bumps with a central dimple | Molluscum contagiosum | No pain | Doctor looks at the skin |
| Fine burrows, itch at night | Scabies | Intense itch | Doctor looks at the skin |
| Lice or eggs in the pubic hair | Pubic lice | Itch | Visible to the naked eye |
| Red, shiny glans | Balanitis or candida | Itch or burning | Doctor looks, sometimes a swab |
| Row of white bumps on the rim | Pearly papules, not an STI | Neither | Nothing, there is nothing to find |
This table is exactly why I think "just look and see whether it is an STI" is a weak method. Four rows end with a doctor looking. And the two most diagnosed STIs in the Netherlands do not even appear in it.
Which STI causes itching?
Itching belongs mainly to scabies, pubic lice and trichomonas, and alongside those to candida, which is not an STI. Herpes feels burning or painful rather than itchy. Genital warts usually do not itch. So itching without a visible bump points more towards a mite, a louse or a fungus.
In women, itching with changed discharge can also belong to bacterial vaginosis, and that is not an STI either.
Other causes are covered in itching and irritation down there.
When is a bump not an STI?
A bump is more often not an STI than people expect. Ingrown hairs, sebaceous glands, pearly papules, a spot and irritation from shaving or soap all produce pictures that resemble one. A single bump that is gone again within a few days rarely fits an STI.
There is simply no feature that lets you rule it out at home with certainty.
If something stays longer than a week or two, changes shape, or gains a sore, a look from a doctor beats another search. A comparison by shape is set out in a bump or blister on your genitals.
Which test finds what you can see?
That depends on the picture. Syphilis, HIV and hepatitis are detected in blood. Chlamydia, gonorrhoea and trichomonas through a PCR on urine or a swab. Herpes through a PCR from an open blister. Warts, molluscum, scabies and pubic lice are identified by looking.
According to Thuisarts.nl, many STIs run without symptoms, which can make a test the only way to gain clarity (thuisarts.nl). The reverse holds just as well: a visible bump is far from always an STI.
What helps me most with these questions is starting at the contact rather than at the bump. If you know when it was and what kind of contact it was, you know which test can find anything at all.
Chlamydia, gonorrhoea and trichomonas can be checked in one go with the chlamydia, gonorrhoea and trichomonas check. If the picture fits herpes instead, HSV-2 in blood tells you something different from a PCR of a blister. Which test suits which situation is covered in the nine STIs that occur in the Netherlands.
If you are unsure about what you see, have your GP take a look at it.
Sources
- RIVM, Thermometer seksuele gezondheid, first half of 2025 (2025) (rivm.nl)
- Groves MJ. Genital Herpes: A Review. American Family Physician (2016). PMID 27281837
- Peeling RW, Mabey D, Kamb ML, Chen XS, Radolf JD, Benzaken AS. Syphilis. Nature Reviews Disease Primers (2017). PMID 29022569
- Chen X, Anstey AV, Bugert JJ. Molluscum contagiosum virus infection. The Lancet Infectious Diseases (2013). PMID 23972567
- Chandler DJ, Fuller LC. A Review of Scabies: An Infestation More than Skin Deep. Dermatology (2019). PMID 30544123
- Coates SJ, Thomas C, Chosidow O, Engelman D, Chang AY. Ectoparasites: Pediculosis and tungiasis. Journal of the American Academy of Dermatology (2020). PMID 31306729
- Tuddenham S, Hamill MM, Ghanem KG. Diagnosis and Treatment of Sexually Transmitted Infections: A Review. JAMA (2022). PMID 35015033
- Thuisarts.nl, Soa (2024) (thuisarts.nl)
Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.
Author
Discreettest
Dr. Naimi, BIG-registered physician, oversees the medical standards behind our content and assessments. Read our medical policy
