2. Skin Infections and Infestations

Learning Objectives

• Classify skin infections by causative organism (bacterial, fungal, viral, parasitic) and name the classic example of each
• Distinguish impetigo, folliculitis, and cellulitis by depth of tissue involvement and clinical appearance
• Recognize the characteristic lesions of tinea corporis, candidiasis, herpes simplex, and varicella-zoster
• Identify scabies and pediculosis from history and examination findings
• Select the appropriate first-line diagnostic test (Gram stain, KOH prep, Tzanck smear, skin scraping) for a given presentation
• Apply correct first-line treatment for each major skin infection and infestation

Quick Answer

Skin infections and infestations are conditions where an organism — bacteria, fungus, virus, or parasite — invades or colonizes the skin and produces a recognizable clinical picture. They matter because they are extremely common, usually diagnosable by inspection alone, and nearly always curable with the right targeted therapy. The organism determines everything that follows: bacterial infections like impetigo and cellulitis need antibiotics, fungal infections like tinea need antifungals, viral infections like herpes need antivirals, and infestations like scabies need anti-parasitic agents. Getting the classification right is the single most important step, because treating a fungal infection with a steroid (or a viral one with an antibacterial) makes things worse, not better. This is one of the most frequently tested topics on USMLE Step 2 CK because the physical exam findings are so distinctive.

This page is for educational purposes. Always verify with current clinical guidelines.

Classifying Skin Infections by Organism

The fastest way to build a differential for any skin lesion is to ask: what kind of organism causes this pattern, and how deep does it go?

This classification is more than an organizing trick — it directly predicts the diagnostic test and the drug class you will reach for, which is exactly how these questions are tested.

Bacterial Infections

Impetigo

Impetigo is a superficial bacterial infection of the epidermis, caused most often by Staphylococcus aureus and sometimes Streptococcus pyogenes. It is the classic "honey-colored crust" disease — thin, fragile vesicles rupture and dry into a golden-yellow crust, typically around the nose and mouth of a school-age child. It spreads by direct contact and is highly contagious in daycare and school settings.

• Cause: S. aureus (most common) or S. pyogenes
• Appearance: Honey-colored crusted erosions, non-scarring
• Treatment: Topical mupirocin for localized disease; oral antibiotics (e.g., cephalexin or amoxicillin-clavulanate) for widespread or bullous impetigo
• Why it matters: Untreated streptococcal impetigo is a recognized trigger for post-streptococcal glomerulonephritis, so it is not "just a skin thing" — a common exam trap.

Folliculitis

Folliculitis is inflammation centered on a hair follicle, most often from S. aureus but occasionally from Pseudomonas (classic "hot tub folliculitis") or fungi. It presents as small, tender, erythematous papules or pustules, each pierced by a hair. Warm compresses and topical antibiotics resolve most cases; oral antibiotics are reserved for extensive or recurrent disease.

Cellulitis

Cellulitis is a deeper bacterial infection involving the dermis and subcutaneous tissue, usually from Streptococcus pyogenes or Staphylococcus aureus entering through a break in the skin. Unlike impetigo, the borders are poorly demarcated, and the patient often has systemic signs — fever, malaise, and sometimes lymphangitic streaking. This distinction (superficial and sharply marginated vs. deep and diffuse) is exactly how erysipelas is separated from cellulitis on exam, and how cellulitis is separated from impetigo.

• Symptoms: Warmth, redness, swelling, pain, fever, poorly defined margins
• Treatment: Oral cephalexin or clindamycin for uncomplicated cases; IV antibiotics and MRSA coverage (e.g., vancomycin) if the patient is septic or has risk factors for MRSA
• Red flag: Rapidly spreading pain out of proportion to exam findings should raise concern for necrotizing fasciitis, a surgical emergency, not simple cellulitis.

Fungal Infections

Tinea Corporis (Ringworm)

Tinea corporis is a dermatophyte infection (commonly Trichophyton rubrum) confined to the stratum corneum — fungi cannot survive in living tissue, only dead keratin, which is why lesions never involve deep structures. It produces the classic annular ("ringworm") plaque: a scaly, erythematous, advancing border with central clearing. Diagnosis is confirmed with a KOH preparation showing branching septate hyphae. First-line treatment is a topical azole (clotrimazole) or allylamine (terbinafine); oral terbinafine is reserved for widespread, scalp, or nail involvement.

Candidiasis

Candida albicans is a yeast, not a dermatophyte, and it thrives in warm, moist environments rather than dry keratin — this is why candidiasis classically appears in skin folds (intertrigo), under the breasts, in the groin, and in the mouth (thrush), while tinea prefers drier, exposed skin. The rash is beefy red with satellite pustules at the periphery, a finding that distinguishes it from tinea on inspection alone. Treatment is a topical antifungal (nystatin or clotrimazole), plus keeping the area dry, since moisture is the underlying driver.

Viral Infections

Herpes Simplex Virus (HSV)

HSV-1 classically causes oral/perioral lesions and HSV-2 classically causes genital lesions, though either type can infect either site through oral-genital contact. The hallmark is a cluster of painful, grouped vesicles on an erythematous base that ulcerate and crust. A Tzanck smear shows multinucleated giant cells, though PCR is now the preferred confirmatory test. HSV establishes lifelong latency in sensory ganglia and reactivates with stress, illness, or sun exposure — it is never cured, only suppressed. Treatment is oral acyclovir, valacyclovir, or famciclovir, started as early as possible.

Varicella-Zoster Virus (VZV)

Primary VZV infection causes chickenpox — diffuse, itchy vesicles in different stages of healing ("dew drops on a rose petal") across the trunk and face. The virus then becomes latent in dorsal root ganglia, and reactivation later in life produces herpes zoster (shingles): a painful, unilateral, dermatomal vesicular rash that respects the midline because it follows a single sensory nerve root. Treatment is supportive for varicella in healthy children; oral acyclovir or valacyclovir is used for zoster, ideally within 72 hours of rash onset to reduce the risk of postherpetic neuralgia.

Parasitic Infestations

Scabies

Scabies is caused by the mite Sarcoptes scabiei var. hominis, which burrows into the epidermis and lays eggs, triggering a delayed hypersensitivity reaction. The hallmark is intense itching that is worse at night, with thin, thread-like burrows found in the finger webs, wrists, and groin. Diagnosis can be confirmed with a skin scraping showing mites, eggs, or fecal pellets under microscopy, though it is often diagnosed clinically. Treatment is topical permethrin 5% cream applied from the neck down, or oral ivermectin for crusted (Norwegian) scabies or outbreak settings — and all close contacts must be treated simultaneously, or the infestation simply bounces back.

Pediculosis

Pediculosis is infestation with lice — Pediculus humanus capitis (head), Pediculus humanus corporis (body), or Phthirus pubis (pubic). Diagnosis is made by finding live lice or nits (eggs) firmly attached to hair shafts, distinguishing them from dandruff, which flakes off easily. Treatment is permethrin or pyrethrin-based shampoos, plus mechanical nit removal and washing of bedding and clothing in hot water.

Diagnosis

The diagnostic test should always match the suspected organism class:

• Gram stain and culture — bacterial infections, especially before starting antibiotics in severe cellulitis
• KOH preparation — fungal infections; shows hyphae (dermatophytes) or pseudohyphae/budding yeast (Candida)
• Tzanck smear or viral PCR — HSV and VZV; PCR is now preferred for sensitivity and speed
• Skin scraping under mineral oil — scabies, to visualize mites, eggs, or fecal pellets
• Wood's lamp — occasionally useful for certain dermatophytes and erythrasma (coral-red fluorescence)

Management

Treatment must be matched to the organism, not the symptom — an itchy rash treated with a topical steroid before the cause is identified can dramatically worsen a fungal infection (tinea incognito) or mask scabies.

1. Bacterial — topical or oral antibiotics targeted to Staphylococcus and Streptococcus, escalating to IV therapy for systemic signs
2. Fungal — topical azoles/allylamines for localized disease; oral antifungals for scalp, nail, or extensive involvement
3. Viral — oral antivirals (acyclovir family) started early; supportive care for self-limited cases like chickenpox in healthy children
4. Parasitic — topical permethrin or oral ivermectin, with simultaneous treatment of close contacts and environmental decontamination (washing bedding/clothing)

Prevention

• Good hand hygiene and avoiding sharing personal items (towels, razors, combs) limits spread of impetigo, tinea, and lice
• Keeping skin folds dry reduces candidal intertrigo
• Prompt treatment of index cases and contacts prevents scabies and lice outbreaks in households and schools
• Varicella and zoster vaccination prevent primary chickenpox and reduce shingles incidence/severity in older adults

Key Terms

Term	Definition	Related Concept
Impetigo	Superficial epidermal bacterial infection with honey-colored crusting	Post-streptococcal glomerulonephritis
Cellulitis	Bacterial infection of dermis and subcutaneous tissue with diffuse, poorly demarcated borders	Erysipelas, necrotizing fasciitis
Erysipelas	Superficial cellulitis with sharply demarcated, raised borders, usually S. pyogenes	Cellulitis (deeper, diffuse)
Dermatophyte	Fungus that digests keratin and is confined to the stratum corneum	Tinea corporis, KOH prep
KOH preparation	Microscopy technique that dissolves keratin to reveal fungal hyphae	Fungal infection diagnosis
Tzanck smear	Cytology test showing multinucleated giant cells in herpesvirus infections	HSV, VZV
Satellite lesions	Small pustules surrounding a main lesion, classic for candidiasis	Intertrigo
Dermatomal rash	Rash confined to the distribution of a single sensory nerve root	Herpes zoster
Burrow	Thin, thread-like tunnel in the epidermis created by the scabies mite	Sarcoptes scabiei
Nit	Louse egg cemented to a hair shaft; does not flake off like dandruff	Pediculosis
Postherpetic neuralgia	Persistent nerve pain after a zoster outbreak resolves	Herpes zoster complications
Permethrin	Topical insecticide/acaricide used for both scabies and lice	Scabies, pediculosis treatment

Common Mistakes

Misconception: Any circular, scaly, red rash on the skin is "ringworm" and should get an antifungal. Why it's wrong: Nummular eczema, pityriasis rosea, and granuloma annulare can all mimic tinea's annular appearance, and applying an antifungal to a non-fungal rash simply delays correct treatment. Worse, applying a topical steroid to unrecognized tinea produces "tinea incognito" — a disguised, spreading infection with a blunted border. Correct understanding: Confirm with a KOH preparation before committing to an antifungal, especially if the rash does not respond to initial treatment within one to two weeks.

Misconception: Cellulitis and impetigo are basically the same thing, just different severities of a skin infection. Why it's wrong: They involve different depths of tissue. Impetigo is confined to the epidermis and is superficial, crusted, and non-systemic. Cellulitis extends into the dermis and subcutaneous fat and commonly causes fever and systemic illness. Correct understanding: Depth of involvement — not just severity — determines the diagnosis, the treatment route (topical vs. systemic antibiotics), and the complications to watch for.

Misconception: Scabies only happens in people with poor hygiene, so a well-groomed patient with itching can be ruled out. Why it's wrong: Scabies transmission depends on close skin-to-skin contact, not cleanliness. It spreads easily in households, dormitories, and nursing homes regardless of hygiene status. Correct understanding: Consider scabies in anyone with intensely pruritic, nocturnally worsening lesions in the finger webs, wrists, or groin, and treat all close contacts simultaneously even if they are asymptomatic, since the incubation period can be several weeks.

Comparison and Connections

Feature	Impetigo	Cellulitis	Tinea Corporis	Scabies
Organism class	Bacterial	Bacterial	Fungal	Parasitic
Depth	Epidermis only	Dermis/subcutis	Stratum corneum	Epidermis (burrows)
Classic finding	Honey-colored crust	Diffuse warm, tender erythema	Annular plaque, central clearing	Nocturnal itch, finger-web burrows
Confirmatory test	Clinical/Gram stain	Clinical/culture if severe	KOH prep	Skin scraping
First-line treatment	Topical mupirocin	Oral cephalexin	Topical clotrimazole	Topical permethrin

Practice Questions

Recall

1. What organism most commonly causes impetigo, and what is its hallmark clinical finding? Answer guidance: Staphylococcus aureus (Streptococcus pyogenes also common); hallmark is honey-colored (golden) crusted lesions.

2. Name the first-line topical treatment for tinea corporis and the diagnostic test used to confirm it. Answer guidance: Topical clotrimazole (or another azole/allylamine); confirmed with a KOH preparation showing branching septate hyphae.

Understanding

3. Why does candidiasis favor skin folds while tinea corporis favors drier, exposed skin? Answer guidance: Candida is a yeast that thrives in warm, moist environments; dermatophytes digest keratin and prefer dry, keratinized stratum corneum. This difference in organism biology explains their different distributions.

4. Explain why herpes zoster produces a strictly unilateral, dermatomal rash while chickenpox is diffuse. Answer guidance: Zoster results from reactivation of latent VZV in a single dorsal root ganglion, so the rash follows that one sensory nerve's distribution and stops at the midline. Chickenpox is the primary viremic infection, spreading hematogenously to skin all over the body.

Application

5. A 6-year-old has honey-crusted lesions around her mouth that are spreading despite good hygiene. What is the most appropriate next step? Answer guidance: Diagnose impetigo clinically; start topical mupirocin for localized disease, or oral antibiotics (cephalexin or amoxicillin-clavulanate) if lesions are widespread or bullous. Also counsel on hand hygiene to prevent spread to contacts.

6. A patient with a known dermatophyte infection was prescribed a combination antifungal-steroid cream and returns two weeks later with a larger, less-defined rash. What happened, and what should be done? Answer guidance: This is tinea incognito — the steroid suppressed the inflammatory border and allowed the fungus to spread undetected. Stop the steroid, confirm with a fresh KOH prep, and treat with an antifungal alone (topical or oral depending on extent).

Analysis

7. A patient presents with rapidly spreading leg erythema, severe pain out of proportion to exam findings, and systemic toxicity. Why should this raise more concern than typical cellulitis? Answer guidance: Pain disproportionate to visible findings, along with rapid spread and systemic toxicity, are classic red flags for necrotizing fasciitis, a surgical emergency requiring immediate surgical debridement plus broad-spectrum antibiotics — not outpatient oral antibiotics as for uncomplicated cellulitis.

8. Compare the transmission and treatment implications of scabies versus pediculosis in a household setting. Answer guidance: Both require simultaneous treatment of close contacts to prevent reinfestation, but scabies transmission requires prolonged skin-to-skin contact and is treated with permethrin cream applied over the whole body, while lice spread via shared personal items and hair contact and are treated with pediculicide shampoos plus mechanical nit removal and laundering of fomites.

FAQ

Q: Why do doctors avoid prescribing a steroid cream for an itchy rash before knowing the cause? Because steroids suppress the immune response that keeps the underlying process in check. In tinea, this creates tinea incognito, allowing the fungus to spread with a disguised appearance. In scabies, a steroid can temporarily calm the itch while the mite population grows unchecked. Diagnosis should come before treatment whenever the cause is not already clear.

Q: Is impetigo contagious, and how long should a child stay home from school? Yes, impetigo spreads easily through direct contact with lesions or contaminated objects. Most guidelines recommend staying home until 24 hours after starting appropriate antibiotic treatment, or until lesions are crusted over and no longer weeping if antibiotics are delayed.

Q: Can you get scabies from a pet? No. Human scabies is caused by Sarcoptes scabiei var. hominis, a mite that only reproduces on human skin. Pets can carry a related mite variant (causing "mange"), which may cause a transient, self-limited itch in humans but cannot establish a true infestation because it cannot complete its life cycle on non-canine skin.

Q: Why does shingles only happen in people who already had chickenpox? Because shingles is not a new infection — it is reactivation of the same varicella-zoster virus that has been dormant in a sensory ganglion since the original chickenpox infection, sometimes for decades. Anyone who was vaccinated with the live varicella vaccine can technically also develop zoster, since the vaccine strain can establish latency too, though at a much lower rate than wild-type infection.

Q: How do you tell a fungal rash from an eczema flare when both are red and scaly? Look at the pattern and the border. Tinea has an active, scaly, advancing edge with central clearing, forming a ring. Eczema tends to be more diffuse, poorly marginated, and often affects flexural areas symmetrically with a history of atopy. When in doubt, a KOH prep quickly settles the question and prevents mistreating one as the other.

Quick Revision

• Impetigo: superficial, epidermis only, honey-colored crusts, S. aureus/S. pyogenes; treat with topical mupirocin or oral antibiotics
• Cellulitis: deeper (dermis/subcutis), diffuse poorly-marginated erythema with systemic signs; treat with oral or IV antibiotics
• Pain out of proportion to exam findings with rapid spread suggests necrotizing fasciitis — a surgical emergency
• Tinea corporis: dermatophyte confined to stratum corneum, annular plaque with central clearing; confirm with KOH prep
• Candidiasis: yeast, favors moist skin folds, beefy red with satellite pustules — distinguishes it from tinea
• Never apply a topical steroid to an undiagnosed rash — risks tinea incognito or masking scabies
• HSV: painful grouped vesicles, lifelong latency in sensory ganglia, confirmed by PCR or Tzanck smear
• VZV: chickenpox is diffuse and diffuse-vesicular; shingles is a strictly unilateral, dermatomal reactivation
• Scabies: nocturnal itch, burrows in finger webs/wrists/groin; treat all close contacts with permethrin or ivermectin
• Pediculosis: nits firmly attached to hair shafts (unlike dandruff); treat with permethrin/pyrethrin shampoo plus laundering
• Match the diagnostic test to the organism: Gram stain (bacterial), KOH (fungal), Tzanck/PCR (viral), skin scraping (parasitic)
• Treatment must target the organism class — antifungals for fungus, antivirals for virus, anti-parasitics for infestations

Related Topics

Prerequisites: Introduction to Dermatology (skin layers and barrier function), basic microbiology (bacterial, fungal, and viral classification)

Related Topics: Eczema and Dermatitis (differentiating inflammatory from infectious rashes), Infectious Disease (systemic complications of skin infections), Pharmacology (antibiotic, antifungal, and antiviral drug classes)

Next Topics: Eczema and Dermatitis, Psoriasis and Other Dermatoses, Skin Cancers and Precancerous Lesions