Skin infections represent an extraordinarily diverse category of infectious disease, encompassing conditions caused by bacteria, viruses, fungi, and parasites that collectively generate one of the largest volumes of clinical consultations in primary care and emergency medicine. The skin, the largest organ of the human body covering approximately two square meters of surface area and serving as the primary physical barrier between the internal milieu and the external microbial environment, is continuously exposed to the extraordinary biodiversity of environmental microorganisms and harbors its own resident microbiome of commensal bacteria, fungi, and viruses that collectively form the cutaneous microbial ecosystem of the skin microbiome. When the physical integrity of the skin barrier is compromised by trauma, maceration, underlying dermatological conditions, or circulatory insufficiency, or when the virulence of an invading pathogen exceeds the capacity of the skin immune defenses to contain it, clinical infection results in the diverse spectrum of presentations that range from the trivial folliculitis of a shaving irritation to the life-threatening necrotizing fasciitis that can spread through fascial planes with lethal rapidity in susceptible individuals.
The clinical importance of skin infections extends beyond their individual morbidity to encompass their role as reservoirs for community transmission of important bacterial pathogens, their contribution to the global antimicrobial resistance crisis through the massive antibiotic prescribing they generate, and their capacity in immunocompromised individuals to serve as portals of entry for systemic infection with otherwise skin-limited pathogens. Staphylococcus aureus, the most important skin pathogen in clinical medicine, colonizes the anterior nares of approximately thirty percent of the general adult population and the skin of a significant additional proportion, serving as the primary reservoir from which community-acquired and healthcare-associated skin and soft tissue infections arise. The emergence and community dissemination of methicillin-resistant Staphylococcus aureus strains capable of causing severe skin infections in previously healthy individuals without healthcare exposure has transformed the clinical approach to skin infections, requiring empirical coverage of methicillin-resistant strains in many clinical contexts where beta-lactam therapy would previously have been the straightforward first-line choice.
The classification of skin infections by depth of tissue involvement provides a clinically useful framework for understanding both the clinical presentation and the management approach appropriate to each category. Superficial infections involving only the epidermis, including impetigo, folliculitis, and superficial fungal infections, are typically managed with topical treatments when localized and with oral antimicrobials when more widespread, with systemic infectious consequences being uncommon in immunocompetent individuals. Infections involving the dermis and subcutaneous tissue, including cellulitis, erysipelas, and cutaneous abscesses, require systemic antibiotic treatment and in the case of abscesses, incision and drainage. Deep infections involving the fascia and muscle compartments, including necrotizing fasciitis and pyomyositis, represent surgical emergencies requiring immediate operative debridement alongside intensive systemic antimicrobial therapy, with delays in surgical intervention directly correlated with mortality in the most severe presentations.
Bacterial Skin Infections: Impetigo, Cellulitis and Abscesses
Impetigo is the most common bacterial skin infection in children, caused predominantly by Staphylococcus aureus and Streptococcus pyogenes either as individual pathogens or as a mixed infection, and producing the characteristic superficial vesicles and pustules that rupture to leave the golden yellow honey-colored crusted lesions on exposed skin surfaces that are pathognomonic of the condition in its classic non-bullous form. Bullous impetigo, caused exclusively by Staphylococcus aureus strains producing exfoliative toxins that disrupt desmoglein-1 in the superficial epidermis, produces larger, more intact fluid-filled bullae that rupture to leave shallow, shiny erosions without the thick crusting of non-bullous impetigo. Both forms of impetigo are highly contagious through direct contact with infected skin secretions, explaining the outbreaks that characterize impetigo in daycare and school settings, and both respond well to topical mupirocin for localized disease or oral antibiotics for widespread or treatment-resistant presentations.
Cellulitis, the acute spreading infection of the dermis and superficial subcutaneous tissue, is one of the most frequent diagnoses leading to hospitalization in developed countries and is associated with substantial diagnostic uncertainty because its clinical presentation of spreading erythema, warmth, swelling, and tenderness in the absence of a definable collection is mimicked by several non-infectious inflammatory conditions including venous stasis dermatitis, contact dermatitis, deep vein thrombosis, and the erythema of crystal arthropathy, with studies documenting misdiagnosis rates of approximately thirty to thirty-five percent in emergency department cellulitis diagnoses. The microbiological diagnosis of cellulitis is frustrated by the technical difficulty of obtaining adequate specimens from the inflamed tissue without the collections that would allow aspiration or biopsy, with blood cultures positive in fewer than five percent of cases and needle aspiration yielding organisms in only twenty to thirty percent, leaving most cellulitis treated empirically without microbiological confirmation.
The treatment of cellulitis is empirically directed at the streptococcal and staphylococcal pathogens responsible for the great majority of cases, with the choice between beta-lactam antibiotics active against methicillin-susceptible organisms and agents with methicillin-resistant Staphylococcus aureus coverage guided by risk factors for methicillin-resistant strains including prior methicillin-resistant Staphylococcus aureus infection or colonization, close contact with methicillin-resistant carriers, injection drug use, and residence in geographic regions with high community methicillin-resistant Staphylococcus aureus prevalence. The clinical markers that distinguish uncomplicated cellulitis from the deeper necrotizing infections that require urgent surgical intervention include the presence of systemic toxicity disproportionate to the local findings, pain disproportionate to the apparent extent of skin involvement, crepitus or gas on imaging suggesting clostridial or mixed anaerobic infection, skin discoloration progressing from erythema through violaceous to frankly necrotic, and the failure to improve with appropriate antibiotic therapy, any of which should prompt urgent surgical consultation and consideration of operative exploration.
Cutaneous abscesses, localized collections of purulent material within the dermis and subcutaneous tissue, are the most common surgical problem presenting to emergency departments in many developed countries and have become dramatically more prevalent with the emergence of community-associated methicillin-resistant Staphylococcus aureus as the predominant uropathogen of skin and soft tissue abscesses. The treatment of cutaneous abscesses is primarily surgical through incision and drainage, which provides immediate pain relief, removes the purulent contents that serve as a bacterial growth medium resistant to antibiotic penetration, and disrupts the fibrin loculations that prevent immune cell access to the infected cavity. The role of adjunctive antibiotic therapy following adequate incision and drainage of uncomplicated abscesses in immunocompetent individuals remains debated, with the NEJM randomized trial of trimethoprim-sulfamethoxazole following incision and drainage demonstrating a reduction in treatment failure and new skin infection within thirty days compared to placebo, supporting the routine addition of a methicillin-resistant Staphylococcus aureus-active oral antibiotic after incision and drainage.
Viral Skin Infections: Herpes and Varicella Zoster
Herpes simplex virus infections of the skin and mucous membranes represent some of the most prevalent viral conditions encountered in dermatological and primary care practice, with herpes simplex virus type 1 infecting approximately sixty to seventy percent of the global adult population as a lifelong infection that persists in the trigeminal dorsal root ganglia following primary oral infection and periodically reactivates to produce the recurrent herpes labialis of the perioral skin and lips familiar to a majority of the affected population as cold sores. Herpes simplex virus type 2 infection, predominantly transmitted through sexual contact and persisting latently in the sacral dorsal root ganglia, causes genital herpes with recurrent ulcerative lesions of the genital skin and mucosa, establishing one of the most prevalent sexually transmitted infections globally with an estimated five hundred million people living with genital herpes simplex virus type 2 infection worldwide.
The clinical recognition of herpes simplex virus skin infections is based on the characteristic morphology of grouped vesicles on an erythematous base that progress through pustulation to crusting and healing, occurring in a characteristic anatomical distribution in primary and recurrent episodes with a prodrome of tingling, burning, or itching at the affected site that precedes vesicle formation in recurrent disease by hours to one to two days. The diagnosis is confirmed by viral swab from the base of a fresh vesicle submitted for polymerase chain reaction testing, which provides the highest sensitivity and specificity of any diagnostic method and can distinguish herpes simplex virus type 1 from type 2, information with implications for prognosis and counseling because type 2 recurs more frequently in the genital region than type 1. Antiviral therapy with aciclovir, valaciclovir, or famciclovir reduces the duration and severity of primary herpes simplex virus episodes, accelerates healing of recurrent episodes when initiated during the prodrome or early lesion stage, and as daily suppressive therapy reduces recurrence frequency by approximately seventy to eighty percent in patients with frequent recurrences.
Herpes zoster, the reactivation of varicella zoster virus that has remained latent in dorsal root ganglia following primary varicella infection, produces the dermatomal rash of shingles characterized by grouped vesicles following the distribution of one or more adjacent dermatomes in a unilateral pattern that does not cross the midline, preceded by a prodrome of dermatomal pain, burning, or hyperesthesia that may precede the rash by days to over a week and that in the absence of the rash can produce diagnostic confusion with musculoskeletal, cardiac, abdominal, or neurological conditions depending on the affected dermatome. The most clinically significant complication of herpes zoster is postherpetic neuralgia, the chronic neuropathic pain in the distribution of the affected dermatome that persists after the acute rash has resolved and that occurs in approximately ten to eighteen percent of all herpes zoster patients with dramatically higher rates in the elderly, producing a potentially severe and treatment-refractory pain syndrome that can last months to years and profoundly impairs quality of life. Antiviral therapy with high-dose valaciclovir or famciclovir initiated within seventy-two hours of rash onset reduces the duration and severity of acute herpes zoster, accelerates skin lesion healing, and most importantly reduces the risk and severity of postherpetic neuralgia, providing the primary rationale for prompt antiviral treatment in all patients with herpes zoster regardless of rash severity.
Fungal Skin Infections: Dermatophytes and Candida
Fungal skin infections, caused predominantly by dermatophyte fungi of the genera Trichophyton, Microsporum, and Epidermophyton and by Candida species, are among the most prevalent infectious conditions affecting human skin globally, with tinea pedis alone estimated to affect fifteen to twenty-five percent of the adult population worldwide. The dermatophytes, specialized fungi uniquely adapted to colonizing keratinized tissues including skin, hair, and nails from which they derive nutrition through the elaboration of keratinase enzymes, produce the various clinical presentations of ringworm classified by anatomical location as tinea capitis of the scalp, tinea corporis of the trunk and limbs, tinea cruris of the groin, tinea pedis of the feet, tinea unguium of the nails, and tinea barbae of the beard area, each with distinctive clinical features and management considerations reflecting the specific anatomical microenvironment of the infection site.
The clinical diagnosis of dermatophyte infections is typically based on the characteristic clinical appearance of a pruritic, erythematous, scaling eruption with a raised, actively advancing border that creates the annular configuration responsible for the lay designation ringworm, though the clinical presentation varies considerably between body sites with the moccasin-type tinea pedis of the soles and sides of both feet looking very different from the typical ringworm pattern and frequently being confused with contact dermatitis or eczema. Potassium hydroxide preparation microscopy of skin scrapings from the active border of lesions, demonstrating the branching septate hyphae of dermatophyte fungi, provides rapid diagnostic confirmation that is sufficiently sensitive and specific to guide treatment decisions in most cases without awaiting fungal culture, though culture on Sabouraud dextrose agar provides species identification that may influence treatment selection and exposure tracing for some infections.
The treatment of cutaneous dermatophyte infections uses topical antifungal agents as first-line therapy for most localized presentations, with the azole antifungals including clotrimazole, miconazole, and ketoconazole and the allylamine antifungals including terbinafine and naftifine providing equivalent clinical efficacy in the majority of randomized trials and both achieving cure rates of eighty to ninety percent for uncomplicated tinea corporis and tinea cruris. Tinea capitis, involving the hair follicles of the scalp from which topical antifungals are excluded by the hair shaft, requires systemic oral antifungal therapy with griseofulvin, terbinafine, itraconazole, or fluconazole for adequate cure, with the duration of treatment varying from six to twelve weeks depending on the agent selected and the causative species identified. Onychomycosis, the fungal infection of nails caused primarily by Trichophyton rubrum and Trichophyton interdigitale, is the most difficult cutaneous fungal infection to cure due to the slow growth of nails that requires extended treatment to allow fungal-free nail to replace infected nail as it grows, with oral terbinafine for six weeks for fingernails and twelve weeks for toenails providing the highest cure rates of approximately seventy to eighty percent at twelve months compared to the substantially lower cure rates of topical nail lacquers used as monotherapy for mild to moderate disease.
Candida skin infections produce a clinically distinct pattern from dermatophyte infections, preferentially affecting the warm, moist, occluded skin folds of the axillae, groin, inframammary folds, and interdigital web spaces, producing an intensely erythematous, exudative, macerated eruption with the satellite papules and pustules peripheral to the main rash that are the most diagnostically distinctive feature of cutaneous candidiasis. Predisposing factors for cutaneous candidiasis include diabetes mellitus, obesity, immunosuppression, antibiotic use that disrupts the normal bacterial colonization protecting against Candida overgrowth, corticosteroid therapy, and any condition producing skin occlusion and maceration. The treatment of cutaneous candidiasis with topical azole antifungals including clotrimazole and miconazole or topical nystatin provides effective clearance of most localized presentations within two to four weeks, with oral azole therapy reserved for extensive or treatment-refractory disease and for immunocompromised patients in whom the risk of invasive candidiasis warrants more aggressive systemic treatment.