Skin Infections — Bacterial, Fungal & Viral Causes, Diagnosis & Treatment — Symptoms, Causes & Treatment | MyMedicPlus

Skin infections are caused by pathogenic microorganisms — bacteria, fungi, viruses, or parasites — breaching the physical barrier of the skin or colonising disrupted skin. The skin is the body's largest organ and most important physical defence; any disruption through cuts, abrasions, insect bites, eczema, or impaired vascular supply (diabetes, peripheral vascular disease) creates an entry point for pathogens. Skin infections are among the most common conditions seen in primary care and emergency medicine. They range enormously in severity: from self-limiting superficial infections such as impetigo (which clears in 7-10 days with topical antibiotics) and tinea pedis (athlete's foot — managed with topical antifungals) to rapidly fatal surgical emergencies such as necrotising fasciitis and streptococcal toxic shock syndrome. The most important clinical skill in skin infections is recognising the rare but life-threatening deep tissue infections — necrotising fasciitis, gas gangrene (Clostridium perfringens), and Fournier's gangrene — which require emergency surgical debridement within hours to prevent mortality. Bacterial skin infections are most commonly caused by Staphylococcus aureus (including methicillin-resistant MRSA) and Group A Streptococcus (Streptococcus pyogenes). Fungal skin infections (dermatomycoses) are extremely common — tinea pedis (athlete's foot) affects up to 25% of the population. Viral skin infections include herpes simplex (HSV-1 and HSV-2), herpes zoster (shingles — varicella-zoster virus reactivation), and molluscum contagiosum. Accurate diagnosis determines the correct treatment class — a fungal infection will not respond to antibiotics.

Bacterial skin infections: Impetigo: highly contagious superficial infection predominantly affecting children; caused by S. aureus (bullous impetigo) or Group A Streptococcus (non-bullous impetigo — the most common type); spreads by direct contact. Cellulitis: acute diffuse infection of the lower dermis and subcutaneous tissue; most commonly caused by Group A, C, or G Streptococcus and S. aureus; typically enters through a skin break — tinea pedis, leg ulcers, trauma, bites, venous cannula sites, lymphoedema (a major predisposing factor — lymphoedema of the lower limb is present in approximately 50% of recurrent cellulitis cases); diabetes and obesity are significant risk factors. Erysipelas: a superficial variant of cellulitis with distinct raised demarcated borders; caused almost exclusively by Group A Streptococcus; classically presents with systemic features (fever, rigors). Folliculitis, furuncles (boils), and carbuncles: S. aureus infection of hair follicles — folliculitis (superficial); furunculosis (deep involvement); carbuncle (multiple coalescing furuncles with multiple discharge points). Community-acquired MRSA (CA-MRSA): increasingly important cause of skin and soft tissue infections — presents as recurrent furunculosis or abscesses, often without hospital exposure. Necrotising fasciitis (NF): surgical emergency — Type 1 (polymicrobial — gram-positive, gram-negative, and anaerobes — most common; occurring in diabetics, immunocompromised, post-abdominal surgery); Type 2 (monomicrobial — Group A Streptococcus — can affect healthy individuals; associated with streptococcal toxic shock syndrome with 30-70% mortality). Fungal skin infections: Tinea (dermatophytosis): caused by dermatophyte fungi (Trichophyton, Microsporum, Epidermophyton) — tinea pedis (foot), tinea corporis (body — ringworm), tinea cruris (groin — jock itch), tinea capitis (scalp — predominantly children; causes inflammatory kerion or non-inflammatory endothrix infection — requires systemic griseofulvin or terbinafine), tinea unguium (onychomycosis — nail). Candidiasis: Candida albicans — intertriginous candidiasis (skin folds — axilla, groin, inframammary — in diabetes and obesity), oral candidiasis (thrush — immunosuppressed, inhaled corticosteroid use, denture wearers), vulvovaginal candidiasis, perleche (angular cheilitis). Viral skin infections: Herpes simplex (HSV-1 and HSV-2): recurrent cold sores (HSV-1) and genital herpes (HSV-2 predominantly); primary infection often more severe; reactivation from dorsal root ganglia triggered by UV light, stress, fever, illness. Herpes zoster (shingles — VZV reactivation): risk increases with age and immunosuppression; lifetime risk 30%; 3.4-5 cases per 1,000 person-years. Molluscum contagiosum: poxvirus — self-limiting umbilicated papules in children and immunosuppressed adults. Risk factors for skin infections: diabetes mellitus (impaired neutrophil function, neuropathy, vascular disease); obesity (skin-fold maceration, lymphoedema); immunosuppression (HIV, corticosteroids, chemotherapy); eczema/psoriasis (disrupted skin barrier); peripheral vascular disease; lymphoedema; intravenous drug use; poverty and overcrowding (impetigo, scabies, tinea).

Impetigo: golden-yellow crusted lesions (honey-coloured crusts — classic non-bullous impetigo from Group A Streptococcus); bullous impetigo (S. aureus exfoliatin toxin causes flaccid bullae); perioral, perinasal, and extremity distribution; mild or absent systemic features; contagious — rapid spread in schools. Cellulitis: spreading erythema (redness), warmth, swelling, and tenderness of the affected area (typically a lower limb — bilateral cellulitis is rare and should prompt consideration of alternative diagnoses such as venous eczema or lipodermatosclerosis); poorly defined borders (distinguishing from erysipelas which has a sharp elevated border); systemic features — fever above 38°C, rigors, tachycardia; regional lymphadenopathy (local lymph node tenderness and swelling — indicates lymphatic involvement); raised inflammatory markers (CRP, WBC). Eron Classification of cellulitis severity: Class I — no systemic signs; Class II — systemic signs but no serious co-morbidity; Class III — systemically ill or with significant co-morbidity; Class IV — limb or life-threatening. Necrotising fasciitis (emergency): pain out of proportion to skin findings (the classic early warning sign — severe pain before visible skin changes); initial erythema progresses rapidly to dusky grey-brown discolouration with blistering, bullae, crepitus (gas in tissues — crackling sensation on palpation), skin necrosis, and anaesthesia of the affected area; systemic features: extreme toxicity, fever, hypotension, confusion (septic shock); the Laboratory Risk Indicator for Necrotising Fasciitis (LRINEC) score uses CRP (above 150), WBC (above 25), haemoglobin, sodium, creatinine, and glucose to stratify NF risk — score above 6 suggests NF. Herpes zoster (shingles): prodromal phase — 2-4 days of pain, burning, hyperaesthesia, or pruritus in a dermatomal distribution before visible rash; grouped vesicles on erythematous base, strictly unilateral and dermatomal; thoracic dermatomes most common (50%); ophthalmic shingles (HZO — V1 branch of trigeminal nerve) risks corneal involvement and visual loss — ophthalmological emergency. Postherpetic neuralgia (PHN): neuropathic pain persisting above 3 months after shingles rash — the most feared complication — affects 10-15% of all shingles cases; rising sharply with age (above 40% in patients above 70).

Diagnosis of most superficial skin infections is clinical. Cellulitis: clinical diagnosis — mark the leading edge of erythema with a skin pen at the time of assessment and review for progression at 24-48 hours (spreading beyond the line indicates treatment failure or inadequate therapy). Blood cultures: recommended for Class III-IV cellulitis and immunocompromised patients — yield is low (2-4%) in uncomplicated cellulitis. Inflammatory markers (CRP, ESR, WBC) — elevated in bacterial infection; CRP above 150 in NF (LRINEC score). Swabs: wound swabs, pus swabs, nose, groin, and axilla swabs for MRSA screening in recurrent or hospital-acquired infections. Imaging for necrotising fasciitis: CT of the affected area (the investigation of choice in clinically suspected NF) — shows gas tracking along fascial planes, fascial thickening, deep fluid, and soft tissue gas; MRI has higher sensitivity but takes longer; X-ray (gas in soft tissues — Clostridium gas gangrene); surgical exploration is the only definitive method to confirm NF when clinical suspicion is high — do not delay surgery for imaging in a patient with clinical NF features. Fungal skin infections: skin scraping (KOH preparation) — dissolves keratin revealing fungal hyphae; Wood's lamp (UV lamp — Microsporum canis fluoresces green; Malassezia furfur — tinea versicolor — fluoresces orange-yellow); fungal culture on Sabouraud agar for speciation (particularly for tinea capitis — species determine treatment duration and systemic therapy choice). Dermoscopy: distinguishes tinea (filamentous structures), molluscum (four-leaf clover), and viral warts. Viral skin infections: Herpes zoster — clinical diagnosis in a typical dermatome; PCR swab of vesicle fluid (gold standard for laboratory confirmation, particularly in atypical presentations or immunocompromised patients). HSV PCR for genital herpes (swab from ulcer base). Tzanck smear (multinucleated giant cells — rapid but non-specific; largely replaced by PCR). Bacterial skin swab culture and sensitivity (C&S): essential for abscess drainage samples and atypical presentations to guide antibiotic selection and detect MRSA.

Impetigo: topical hydrogen peroxide 1% cream (Crystacide) — NICE recommended first-line for localised non-bullous impetigo (non-inferior to fusidic acid, without resistance concerns); topical fusidic acid 2% (second-line where hydrogen peroxide fails); topical mupirocin 2% (for MRSA-positive impetigo); oral antibiotics (flucloxacillin 500 mg QDS for 7 days, or cefalexin as alternative) for extensive, bullous, or treatment-failing impetigo. Cellulitis: oral antibiotics for Class I-II (Eron Classification): flucloxacillin 500-1000 mg QDS (7-day course; first-line for staphylococcal and streptococcal coverage); cefalexin 500 mg QDS (alternative — better tolerated); co-amoxiclav for bite-associated cellulitis or diabetic foot infections (adds anaerobic and gram-negative coverage); clarithromycin or doxycycline for penicillin-allergic patients. Intravenous antibiotics for Class III-IV: IV benzylpenicillin (for pure streptococcal cellulitis) or IV flucloxacillin 1-2 g QDS; or IV co-amoxiclav (for polymicrobial, diabetic foot, or cat/dog bite); add IV clindamycin or metronidazole for suspected anaerobic involvement (severe NF). MRSA skin and soft tissue infections: oral doxycycline 100 mg BD, or co-trimoxazole (trimethoprim-sulfamethoxazole); for severe infections: IV vancomycin (targeting trough 15-20 mg/L), or IV daptomycin (6 mg/kg daily). Incision and drainage (I&D): the primary treatment for cutaneous abscesses, furuncles, and carbuncles — antibiotic therapy alone is insufficient for fluctuant abscesses. Necrotising fasciitis: immediate emergency surgical debridement is the cornerstone — wide excision of all necrotic tissue; often requires multiple return trips to theatre; broad-spectrum IV antibiotics: piperacillin-tazobactam 4.5 g TDS plus clindamycin 600 mg TDS (clindamycin reduces streptococcal toxin production — IDSA recommended for Group A Streptococcal NF) plus vancomycin for MRSA coverage; IV immunoglobulin (IVIG) for streptococcal toxic shock syndrome (STREP-IVIG-BLISS trial supports use); skin grafting and reconstructive surgery after infection controlled. Fungal infections: tinea pedis (athlete's foot) — topical terbinafine 1% cream for 1 week or topical clotrimazole 1% cream BD for 4 weeks; tinea unguium (nail) — oral terbinafine 250 mg daily for 6 weeks (fingernails) to 12-16 weeks (toenails) — superior to topical amorolfine alone; tinea capitis — oral griseofulvin (first-line for Microsporum canis — 10-20 mg/kg daily for 8-12 weeks) or oral terbinafine (first-line for Trichophyton tonsurans); oral fluconazole 150-300 mg weekly for 4 weeks for tinea corporis in immunosuppressed or extensive disease. Candidal skin infections: topical clotrimazole or miconazole; oral fluconazole 150 mg single dose for vulvovaginal candidiasis; address predisposing factors (diabetes control, drying skin folds, inhaler spacer use). Herpes zoster (shingles): oral aciclovir 800 mg 5 times daily for 7 days (most effective if started within 72 hours of rash onset); valaciclovir 1 g TDS or famciclovir 500 mg TDS (improved bioavailability; once or thrice daily — better adherence); ophthalmic HZO: urgent ophthalmology referral plus topical ophthalmic antiviral (aciclovir eye ointment); postherpetic neuralgia: amitriptyline, gabapentin, or pregabalin; topical lidocaine 5% patches or capsaicin 8% patch. Shingles vaccine: recombinant zoster vaccine (RZV — Shingrix) — two doses given 2-6 months apart; offered to all adults above 70 in the UK (JCVI recommendation); 97% efficacy against shingles in 50-69-year-olds and 89% in those above 70; 89% efficacy against PHN.

Necrotising fasciitis and sepsis: the most feared and life-threatening complication of bacterial skin infection — occurs primarily in deep tissue bacterial infections (Group A Streptococcus, polymicrobial — and also from spreading from inadequately treated cellulitis); overall mortality 25-40%; higher in delayed diagnosis, immunocompromised, and extensive disease; requires ICU-level supportive care alongside emergency surgery. Post-streptococcal complications: acute rheumatic fever (ARF — from Group A Streptococcal throat infection, rarely skin) — declining in developed countries but still significant in Aboriginal and Pacific Island communities; post-streptococcal glomerulonephritis (PSGN) — can follow both throat and skin Group A Streptococcal infections; presents with haematuria, proteinuria, hypertension, and oedema 1-3 weeks after skin infection. Lymphoedema and recurrent cellulitis: each episode of leg cellulitis causes lymphatic damage, increasing risk of future episodes — a self-reinforcing cycle; approximately 22% of patients have a recurrence within 3 years; prophylactic phenoxymethylpenicillin 250-500 mg BD (continuous or seasonal) reduces recurrence risk by approximately 50% in patients with 2+ episodes per year (PATCH I trial, 2013). Antibiotic resistance: MRSA skin infections require specific antibiotic selection and close follow-up; community-acquired MRSA (Panton-Valentine leukocidin positive — PVL-MRSA) causes severe, recurrent, and necrotising skin infections particularly in young healthy adults — specialist referral and household screening required. Scarring and disfigurement: severe bacterial skin infections (carbuncles, NF, large abscesses) and herpes infections in immunocompromised patients may result in permanent scarring. Postherpetic neuralgia (PHN): severe neuropathic pain persisting beyond 3 months after shingles; affects approximately 10-15% of all shingles cases overall, rising to 40%+ in those above 70; can be debilitating and refractory to treatment; major cause of morbidity in elderly patients following shingles. Herpes zoster ophthalmicus (HZO) complications: keratitis, uveitis, raised intraocular pressure, secondary bacterial infection, corneal scarring, and permanent visual loss if not promptly treated.

Hand hygiene and skin care: thorough handwashing with soap and water for at least 20 seconds reduces skin infection transmission — particularly impetigo, tinea, and viral skin infections; avoid sharing towels, nail clippers, combs, and footwear (reduces tinea spread); use antifungal powder in footwear and dry feet thoroughly (particularly web spaces) after bathing. Wound care: prompt cleaning and covering of skin breaks, cuts, and abrasions with antiseptic and waterproof dressings reduces entry-point infections; complete courses of antibiotic therapy for early skin infections to prevent progression. Cellulitis prevention in high-risk patients: regular emollient application to prevent skin dryness and cracking (particularly in eczema and lymphoedema); prompt treatment and suppression of tinea pedis (the most common entry point for leg cellulitis — treat with topical terbinafine, replace trainers, dry interdigital spaces); compression therapy and graduated compression stockings for lymphoedema (reduces skin tension and bacterial colonisation); prophylactic antibiotics (phenoxymethylpenicillin 250-500 mg BD or erythromycin for penicillin allergy) for patients with 2 or more cellulitis episodes per year — recommended by CREST (Clinical Resource Efficiency Support Team) and British Lymphology Society guidelines. Diabetes control: meticulous glycaemic control (HbA1c target below 48 mmol/mol — 6.5%) reduces skin infection susceptibility — diabetic patients have impaired neutrophil function and vascular supply; daily foot inspection for abrasions, blisters, and interdigital tinea; specialist diabetic foot care. Shingles vaccination: recombinant zoster vaccine Shingrix strongly recommended for adults above 50 (offered NHS to 70-79-year-olds in the UK); dramatically reduces shingles incidence and PHN severity; two doses required. MRSA decolonisation: 5-day decolonisation regimen for known MRSA carriers undergoing elective surgery — nasal mupirocin 2% (3 times daily), chlorhexidine body wash, and chlorhexidine shampoo — reduces MRSA surgical site infection risk.

Go to A&E immediately for: rapidly spreading skin redness with severe pain, particularly in a diabetic or immunocompromised patient — possible necrotising fasciitis (surgical emergency — any delay increases mortality); skin changes with fever, confusion, or hypotension — sepsis from skin source; pain out of proportion to visible skin changes (NF early warning sign); gas or crepitus under the skin; and red streaks spreading up the limb from a wound (ascending lymphangitis — indicates systemic spread). Seek urgent GP or walk-in review (within 24 hours) for: cellulitis with systemic features (fever above 38°C, rigors, worsening despite 2 days of oral antibiotics); cellulitis spreading rapidly or involving the face; rapidly enlarging or painful abscess or boil requiring drainage; shingles rash near the eye (ophthalmic HZO) — same-day referral to ophthalmology; and extensive impetigo in a child requiring systemic antibiotics. See a GP (routine or urgent appointment) for: spreading skin rash that does not respond to over-the-counter antifungals after 2 weeks; suspected nail fungal infection; shingles rash on trunk or limbs for antiviral prescription (most effective within 72 hours); and recurrent boils or skin abscesses — possible MRSA colonisation requiring swabs and decolonisation. Diabetic patients should contact their GP or diabetes care team urgently for any foot wound, blister, or skin infection — early aggressive treatment prevents limb-threatening diabetic foot complications.