Guides

What Is Acne Inversa?

An introductory overview of terminology, course, and context.

60-second summary

If you only remember five things, remember these:

  • Acne Inversa — also called Hidradenitis Suppurativa or HS — is a chronic inflammatory skin condition, not a form of acne and not an infection.
  • It typically causes recurring painful nodules, abscesses, drainage, tunnels and scarring in skin folds such as the armpits, groin and under the breasts.
  • HS is not caused by poor hygiene and is not contagious.
  • Severity varies widely; the Hurley system describes three broad stages, from isolated abscesses to widespread interconnected tunnels.
  • A dermatology practice can assess whether recurring nodules are HS and discuss treatment options.

HS is a lasting condition, not a run of bad luck with boils. Recognising the pattern often shortens the years many people spend without a diagnosis.

The exact cause isn't fully understood. Genetics, smoking, hormones and mechanical friction are discussed as contributing factors, but no single trigger explains every case.

If painful nodules keep recurring at the same sites, note where and how often, and raise that pattern with a dermatologist.

Find HS-experienced care

This summary is written and updated together with the full text. The evidence, limitations and sources stay unchanged in the article below.

Continue to the detailed explanation

Acne Inversa, medically known as Hidradenitis Suppurativa (HS), is a chronic inflammatory skin condition that is more common than many people realise — and is misdiagnosed far too often. Despite the name, it is not a type of acne: it starts deeper in the skin, around hair follicles and apocrine sweat glands in body folds, and follows a very different course.

This page is a plain-language orientation to what HS is, how it can present, and why the name matters. It cannot tell you whether your own symptoms are HS — only a clinical assessment can do that — but it can help you recognise a pattern worth raising in an appointment.

Symptoms of Acne Inversa

Nodules, abscesses, tunnels, scarring, and the body areas most often affected.

 Diagnosis Guide

How a diagnosis is reached and how to prepare for that conversation.

 Is Acne Inversa contagious?

A direct answer to one of the most common and most damaging misconceptions.

 Treatment Options

The treatment classes patients may discuss with a dermatologist.

What HS actually is

HS is classified as a chronic inflammatory skin disease centred on the hair follicle, most often in areas where skin rubs against skin: the armpits, groin, inner thighs, under the breasts, buttocks, and the perianal and lower abdominal folds.

Guideline literature describes the disease process as inflammation and blockage around hair follicles that can progress to abscesses, and — if it recurs in the same area — to tunnels (sinus tracts) under the skin and scarring. That progression is why isolated nodules described only as 'boils' can be years away from an accurate diagnosis.

How common it is, and who it affects

Population-based research places HS among the more common inflammatory skin conditions, though estimates vary by study design and country: German insurance records captured a diagnosis in about 0.03% of people, while studies that examined people found 0.3% to around 1%. It most often begins after puberty and before age 40, and is diagnosed more often in women than in men, though it affects people of any sex.

Family history matters: a substantial share of people with HS report a close relative with the condition, which points to a genetic predisposition — not something 'caught' from another person.

What causes it — and what doesn't

The exact cause is not fully mapped, but current understanding treats HS as an interaction between genetic predisposition, immune-system dysregulation, hormonal factors, and mechanical stress on hair follicles (friction, occlusion). Smoking and higher body weight are strongly associated with HS, but association does not prove that either factor caused an individual's HS, and neither alone explains why it develops. European treatment guidance advises stopping smoking and managing weight for their broader health benefits and possible HS benefit, while noting that randomised trials showing an effect of either on HS severity are lacking.

What HS is not caused by: poor hygiene, an infection you can catch or pass on, or something you did wrong. Bacteria found in HS lesions are now understood as a secondary factor in an already-inflamed environment, not the original cause — which is also why antibiotics are used for their anti-inflammatory effect as much as any antibacterial one.

Severity

The three Hurley stages

Hurley staging is the classic way clinicians describe structural HS — tunnels and scarring — in one specific body region. It's regional and largely static: current inflammatory activity, which can change from week to week, is measured separately with a score called IHS4. Two body areas can sit at different Hurley stages at once, and inflammation can improve even while an established stage stays the same.

Hurley Stage I

No tunnels yet

One or more abscesses without tunnels or significant scarring in this region. Often mistaken for recurring boils — and not the same as "mild disease overall."

Hurley Stage II

Separated tunnels

Recurring abscesses with tunnel formation and scarring in this region, with affected areas still separated by normal skin.

Hurley Stage III

Interconnected tunnels

Diffuse or near-diffuse involvement of this region, with multiple interconnected tunnels and extensive scarring.

Myths

What HS is often mistaken for

These misconceptions are common — and they're a major reason diagnosis is often delayed for years.

"It's just bad acne"

HS starts in a different part of the follicle and behaves differently from acne vulgaris; standard acne treatment isn't the right frame for it.

"It means you're not clean"

Hygiene does not cause HS, and washing more will not resolve it. This misconception drives real shame that isn't warranted by the evidence.

"It's a series of unlucky boils"

A single boil or abscess does not establish HS, and boils from bacterial skin infection can recur for other reasons too. HS becomes more likely when painful or draining lumps keep coming back in characteristic places — armpits, groin, genital or perineal area, buttocks, under the breasts — together with typical nodules, abscesses, tunnels or scarring. No single feature confirms HS on its own; a clinician also considers bacterial infection and simple abscesses.

Quick facts

  • HS usually affects skin folds: armpits, groin, under the breasts, buttocks, inner thighs.
  • It is not contagious and cannot be spread by contact, shared towels, or clothing.
  • Many people wait years for a correct diagnosis.
  • Smoking and higher body weight are strongly associated with HS, though HS occurs at any weight and in people who have never smoked.
  • There is currently no cure, but treatment can reduce flares, pain, and progression.

When not to wait for a routine appointment

  • Fever, spreading redness, or feeling significantly unwell alongside a flare
  • A lesion that is rapidly enlarging, very painful, or not draining safely on its own
  • Pain severe enough to limit sitting, walking, or arm movement

Internal links

Hurley Stages vs IHS4

Why a Hurley stage and an IHS4 score answer different questions — and why treatment can help while your Hurley stage stays the same.

 What Is Hidradenitis Suppurativa?

A focused introduction to the HS/Acne Inversa naming and core facts.

 Acne Inversa, simply explained

A plain-language explainer for people newly encountering the term.

 10 Acne Inversa myths

Common misconceptions about HS, addressed directly.

 How common is HS in Germany?

Why published estimates range from 0.03% to over 1% — and what each study actually measured.

 Symptoms of Acne Inversa

A closer look at lesions, locations, and patterns.

Key sources for this pillar page

Last editorially reviewed:

  1. S2k guideline on the therapy of hidradenitis suppurativa / acne inversa AWMF, 2024
  2. North American clinical management guidelines for hidradenitis suppurativa, Part I Journal of the American Academy of Dermatology, 2019
  3. Hidradenitis suppurativa Nature Reviews Disease Primers, 2020
  4. European S2k guidelines for hidradenitis suppurativa/acne inversa Part 1. Epidemiology, diagnosis and clinical assessment Journal of the European Academy of Dermatology and Venereology, 2026
  5. Development and validation of the International Hidradenitis Suppurativa Severity Score System (IHS4) British Journal of Dermatology, 2017