How does a wart form?
A wart forms when a papillomavirus (HPV) gets in through a tiny scratch and infects the cells at the bottom of the skin. It drives them to multiply and to make too much thick skin, without setting off any alarm. In children, though, about half of warts go away on their own within a year (Bruggink, 2013).
Simplified diagram, not to scale. Dashed curves are estimates; only the 13-week values come from a trial.
How does a wart form?
A wart, seen in cross-section through the skin. Choose a situation, then tap a label on the drawing to learn what it is.
Sources: how the virus gets in, how it is made at the top of the skin, and the defenses (Stanley, Clin Microbiol Rev, 2012); trial of liquid nitrogen, 40% salicylic acid or waiting in 250 people aged 4 to 79 (Bruggink et al., CMAJ, 2010); clearing in children (Bruggink et al., Ann Fam Med, 2013); effect of cold, acid and the black dots (DermNet); size of the virus (ICTV). Drawing not to scale; scene timings are estimates.
Keyboard: 1 to 3 situations, Space pause, L slow motion, R restart, ← → previous or next step.
Frequently asked questions about warts
Are warts contagious?
Yes, warts are contagious, but mostly through close, repeated contact at home and at school. Among about 1,100 Dutch children aged 4 to 12 followed for 11 to 18 months, having a family member with warts nearly doubled the risk of getting them (× 2.1), and every 10% of classmates with warts raised it by 20%; the authors therefore advise focusing prevention on families and classrooms rather than swimming pools (Bruggink, Pediatrics, 2013). Not sharing towels, socks or shoes, and not picking at the wart, helps limit spread (NHS).
How long does a wart last without treatment?
An untreated wart often lasts a few months to two years: many go away on their own within 1 to 2 years (MSD Manual). In three Dutch schools, 1 child in 3 had warts, and about half of those children had none a year later; the youngest healed faster (Bruggink, Ann Fam Med, 2013). In adults, warts usually last longer (DermNet). They go away when the body's defenses finally spot the infected cells.
What are the black dots in a wart?
The black dots in a wart are the tips of tiny blood vessels, capillaries, that grow up into the wart: they are not roots or seeds (DermNet). You see them through the thick skin, especially when you file it down, and they can then bleed in pinpoints. A corn or callus does not have these dots: that is a good way to tell them apart from a plantar wart (DermNet).
Plantar wart or corn: how can you tell them apart?
A plantar wart differs from a corn in three ways: the skin lines stop at the wart, it often shows black dots, and it hurts more when you squeeze it from the sides than when you press on it (DermNet). A corn always sits where something rubs or presses, and it hurts with direct pressure. A wart can grow anywhere on the sole. If in doubt, it is best to show it to a doctor or a podiatrist.
Why does a wart come back after treatment?
A wart often comes back because treatment removes the visible cells but not always the virus hiding in nearby cells at the bottom of the skin. That is why liquid nitrogen is usually repeated every 1 to 2 weeks for 3 to 4 months (DermNet). After removal by electrocautery, about 20% of warts come back within a few months (DermNet). People with weakened defenses, for example after a transplant, have more warts and more recurrences (DermNet).
True or false?
On the hands, freezing works better than salicylic acid.
True. After 13 weeks, 49% of hand warts had cleared with liquid nitrogen, versus 15% with salicylic acid and 8% with no treatment (Bruggink, 2010).
On the soles of the feet, freezing works much better than acid.
False. On the soles, results were close: 30% cleared with liquid nitrogen, 33% with salicylic acid, 23% with no treatment (Bruggink, 2010).
You mostly catch warts at the swimming pool.
False. In children followed over time, swimming pools, shared showers and going barefoot were not linked to warts (van Haalen, 2009). Having a family member with warts roughly doubles the risk (Bruggink, 2013).
This page as text
The same information as the charts, to read without animation: for each chart, the values at the main moments, then the notes with their level of evidence.
How a wart is born, grows and goes away
The virus gets in through a scratch, hides for weeks at the bottom of the skin, then makes the wart grow. The wart goes away when the body's defenses spot it, often within 1 to 2 years. Estimated curves, relative levels.
| Time | Virus in the skin (dotted) | Wart size (dotted) | Defenses targeting it (T cells) (dotted) | What happens |
|---|---|---|---|---|
| Contact | 0 | 0 | 0 | with the virus |
| 3 mo | 50 | 15 | 1 | Month 3 after contact |
| 6 mo | 100 | 76 | 3 | Month 6 after contact |
| 12 mo | 95 | 95 | 30 | Month 12 after contact |
| 18 mo | 20 | 25 | 75 | Month 18 after contact |
| 24 mo | 0 | 0 | 20 | Month 24 after contact |
Key points
- A scratch is enough (Until month 1.5, Shown in humans)
The virus has to reach the cells at the bottom of the skin, which takes a small wound. It needs 24 to 48 hours to get into a cell. Source: Stanley, « Epithelial cell responses to infection with human papillomavirus », Clin Microbiol Rev, 2012 - Nothing raises the alarm (Until month 6, Shown in humans)
The virus does not kill the cells it infects and does not enter the blood. Alarm signals such as interferons are not released. Source: Stanley, « Epithelial cell responses to infection with human papillomavirus », Clin Microbiol Rev, 2012 - Months before you see it (From month 0.5 to month 6, Shown in humans)
It takes at least 3 weeks for new viruses to come out, and often months before a wart can be seen. Source: NHS, « Warts and verrucas », 2023 - Contagious mostly at home and at school (From month 3 to month 15, Shown in humans)
Among about 1,100 children aged 4 to 12, having a family member with warts raised the risk 2.1 times. The authors advise focusing prevention on families and classrooms rather than public places. Source: Bruggink et coll., « Warts transmitted in families and schools: a prospective cohort », Pediatrics, 2013 - The defenses finally see it (From month 12, Shown in humans)
In warts that go away, there is an influx of T cells and macrophages; this has mostly been studied in genital warts. Source: Stanley, « Epithelial cell responses to infection with human papillomavirus », Clin Microbiol Rev, 2012 - Half within a year in children (From month 12, Shown in humans)
In three Dutch schools, 1 child in 3 had warts; about half of them had none a year later. The youngest healed faster. Source: Bruggink et coll., « Natural course of cutaneous warts among primary schoolchildren: a prospective cohort study », Ann Fam Med, 2013 - Estimated durations (Throughout, Hypothesis)
The shape of the curves follows the order of the stages; the months shown are estimates and vary widely from one wart to another.
Wart on the hand: liquid nitrogen, acid or wait?
In a Dutch trial, 116 people with common warts, mostly on the hands, were randomly assigned. Liquid nitrogen clearly wins. Curves: share of people with no warts at all, measured at 13 weeks, with an estimated path before.
| Time | Liquid nitrogen, every 2 weeks (49% at 13 weeks) (dotted) | 40% salicylic acid, every day (15%) (dotted) | Wait without treatment (8%) (dotted) | What happens |
|---|---|---|---|---|
| Start | 0 | 0 | 0 | of the trial |
| 2 wk | 3 | 1 | 0 | Week 2 of the trial |
| 4 wk | 10 | 3 | 2 | Week 4 of the trial |
| 6 wk | 21 | 6 | 3 | Week 6 of the trial |
| 8 wk | 32 | 10 | 5 | Week 8 of the trial |
| 10 wk | 41 | 13 | 7 | Week 10 of the trial |
| 13 wk | 49 | 15 | 8 | Week 13 of the trial |
Key points
- A randomized trial (Throughout, Shown in humans)
250 people aged 4 to 79, randomly assigned to liquid nitrogen, salicylic acid or waiting, at their family doctor's office. Source: Bruggink et coll., « Cryotherapy with liquid nitrogen versus topical salicylic acid application for cutaneous warts in primary care: randomized controlled trial », CMAJ, 2010 - At 13 weeks (From week 10, Shown in humans)
All warts were gone in 49% of people treated with liquid nitrogen, 15% with salicylic acid, and 8% of those who waited. Source: Bruggink et coll., « Cryotherapy with liquid nitrogen versus topical salicylic acid application for cutaneous warts in primary care: randomized controlled trial », CMAJ, 2010 - How the liquid nitrogen was applied (Throughout, Shown in humans)
One session every 2 weeks, 3 applications in a row, until a 2 mm frozen ring formed around the wart (2 to 10 seconds). Source: Bruggink et coll., « Cryotherapy with liquid nitrogen versus topical salicylic acid application for cutaneous warts in primary care: randomized controlled trial », CMAJ, 2010 - More bothersome side effects (Throughout, Shown in humans)
Pain, blisters and scabs were more common after liquid nitrogen than after acid. Source: Bruggink et coll., « Cryotherapy with liquid nitrogen versus topical salicylic acid application for cutaneous warts in primary care: randomized controlled trial », CMAJ, 2010 - Estimated curves (Throughout, Hypothesis)
Only the 13-week values are measured; the path between 0 and 13 weeks is estimated.
Plantar wart: liquid nitrogen, acid or wait?
In the same trial, 124 people had warts on the sole of the foot. Here, the three options give similar results. Curves: share of people with no warts at all, measured at 13 weeks, with an estimated path before.
| Time | Liquid nitrogen, every 2 weeks (30% at 13 weeks) (dotted) | 40% salicylic acid, every day (33%) (dotted) | Wait without treatment (23%) (dotted) | What happens |
|---|---|---|---|---|
| Start | 0 | 0 | 0 | of the trial |
| 2 wk | 2 | 2 | 2 | Week 2 of the trial |
| 4 wk | 6 | 7 | 5 | Week 4 of the trial |
| 6 wk | 13 | 14 | 10 | Week 6 of the trial |
| 8 wk | 19 | 21 | 15 | Week 8 of the trial |
| 10 wk | 25 | 28 | 19 | Week 10 of the trial |
| 13 wk | 30 | 33 | 23 | Week 13 of the trial |
Key points
- At 13 weeks, no clear winner (From week 10, Shown in humans)
All warts were gone in 30% with liquid nitrogen, 33% with acid and 23% while waiting: the gap could be due to chance. Source: Bruggink et coll., « Cryotherapy with liquid nitrogen versus topical salicylic acid application for cutaneous warts in primary care: randomized controlled trial », CMAJ, 2010 - Acid does better than nothing (Throughout, Shown in humans)
Pooling the trials, salicylic acid clears more warts than a product with no effect (1.3 times more on the foot, 2.7 times more on the hand). Source: Kwok et coll., « Topical treatments for cutaneous warts », Cochrane, 2012 - File, then protect the skin around it (Throughout, Shown in humans)
In the trial, the softened wart was filed down before each application and the skin around it was covered. Drugstore products are often 17%. Source: Bruggink et coll., « Cryotherapy with liquid nitrogen versus topical salicylic acid application for cutaneous warts in primary care: randomized controlled trial », CMAJ, 2010 - The hardest to treat (Throughout, Shown in humans)
Warts on the sole of the foot are among the hardest warts to treat. Source: Dinulos, « Verrues » (warts), MSD Manual Consumer Version (French edition), 2025 - Why the foot resists (Throughout, Hypothesis)
Thick skin and the pressure of walking might block the cold and the acid: this is a hypothesis. - Estimated curves (Throughout, Hypothesis)
Only the 13-week values are measured; the path between 0 and 13 weeks is estimated.
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Metabolive. "How does a wart form?" Updated October 29, 2026. https://metabolive.com/en/everyday/warts.
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