Skin lesions have dozens of possible causes, ranging from minor infections and allergic reactions to autoimmune diseases and skin cancer. Nearly every person develops some type of skin lesion during their lifetime. Acne alone affects up to 50 million Americans each year, eczema strikes about 1 in 10, and psoriasis affects roughly 7.5 million people in the United States. Understanding the broad categories of causes can help you recognize what you’re dealing with and whether it needs attention.
Primary vs. Secondary Lesions
Dermatologists classify skin lesions into two broad groups. Primary lesions are the initial change in your skin. A macule, for example, is a flat spot smaller than 10 mm that differs in color from the surrounding skin but isn’t raised or depressed. A vesicle is a small fluid-filled blister under 10 mm. Other primary lesions include bumps, welts, and cysts.
Secondary lesions develop from primary ones over time or through scratching, infection, or healing. Scales are buildups of dead skin cells, common in psoriasis and fungal infections. Crusts (scabs) form from dried serum, blood, or pus and appear in infections like impetigo. Scars, erosions, and areas of changed pigmentation are also secondary. The distinction matters because the type of lesion you see gives clues about what caused it and how far along it is.
Infections: Bacteria, Viruses, and Fungi
Infections are among the most common causes of skin lesions, and the specific pathogen determines what they look like.
Bacterial infections frequently involve Staphylococcus or Streptococcus species. These can produce pus-filled bumps, crusty sores (as in impetigo), or deeper abscesses. Some bacterial skin infections start from a small cut or insect bite that lets bacteria under the skin’s surface.
Viral infections cause a wide range of lesions. Herpes simplex produces clusters of small, painful blisters that recur in the same location. Herpes zoster (shingles) creates a painful, blistering rash along a nerve path. Human papillomavirus causes warts, which are rough, raised growths. Molluscum contagiosum produces small, pearly bumps common in children.
Fungal infections are caused by dermatophytes, a group of fungi that feed on skin, hair, and nail cells. These infections go by different names depending on where they appear: athlete’s foot (feet), jock itch (groin), ringworm (body), and tinea capitis (scalp). Fungal skin lesions typically look red, swollen, or bumpy and often form a ring-shaped rash with a clearer center.
Inflammatory and Autoimmune Conditions
When the immune system misfires, the skin is often the first place it shows up.
Eczema (atopic dermatitis) affects up to 1 in 5 children under 18 and about 1 in 10 Americans overall. It produces itchy, dry, inflamed patches that can crack and weep. Flare-ups are triggered by irritants, allergens, stress, and dry weather.
Psoriasis causes thick, scaly plaques where the body produces skin cells far too quickly. These silvery-white patches most often appear on the elbows, knees, and scalp, though they can develop anywhere. About 7.5 million Americans live with the condition.
Cutaneous lupus is a form of the autoimmune disease lupus that specifically targets the skin. The immune system attacks skin cells, causing chronic inflammation. The hallmark is a butterfly-shaped rash that spreads across both cheeks and the bridge of the nose, though lupus skin lesions can also appear as red, scaly patches elsewhere on the body. On darker skin, the rash may look purple or brown rather than red. UV light exposure typically worsens symptoms, and healed lesions can leave permanent scars or patches of lighter and darker pigmentation.
Contact Dermatitis and Environmental Triggers
Direct contact with irritants or allergens is one of the most straightforward causes of skin lesions. There are two types. Irritant contact dermatitis happens when a substance physically damages the skin. Common culprits include solvents, bleach, detergents, rubber gloves, fertilizers, and certain hair products. Allergic contact dermatitis occurs when a substance triggers an immune reaction in a person who has become sensitized to it.
The most frequent allergens include nickel (found in jewelry, belt buckles, and many everyday metal items), formaldehyde (in preservatives and cosmetics), balsam of Peru (in perfumes, toothpastes, and flavorings), antibiotic creams, hair dyes, and plants like poison ivy and mango that contain a highly irritating substance called urushiol. Children develop reactions from ear-piercing jewelry, diaper materials, baby wipes, and clothing snaps or dyes.
Certain jobs carry higher risk. Agricultural workers, cleaners, construction workers, hair stylists, healthcare workers, mechanics, and florists all have frequent exposure to known triggers. Swimmers and scuba divers can react to the rubber in goggles or face masks.
Medication Reactions
Almost any medication can cause a skin reaction, but certain drugs are far more likely offenders. These drug eruptions take many different forms, which is part of what makes them tricky to identify.
The most common type is a maculopapular eruption, a mildly itchy rash of flat and raised spots that resembles measles. Antibiotics, pain relievers, and sulfonamides are frequent causes. Some medications produce eruptions that look like acne but lack the blackheads and whiteheads typical of true acne. Corticosteroids and lithium are known for this.
Fixed drug eruptions are distinctive: they create well-defined, dusky red or purple spots that reappear in exactly the same location each time you take the offending medication. NSAIDs, antibiotics, and acetaminophen are common triggers.
Photosensitive eruptions occur when a medication makes your skin unusually reactive to sunlight. The result is a burn-like or eczema-like rash on sun-exposed areas. Certain antibiotics (tetracyclines, fluoroquinolones), NSAIDs, and sulfonamides can cause this. More serious drug reactions, though rare, can cause widespread blistering or skin detachment and require immediate medical care.
Skin Cancer and Precancerous Lesions
Not all skin lesions are benign. Skin cancer often first appears as a subtle change that’s easy to dismiss.
Basal cell carcinoma, the most common type, typically looks like a skin-colored or pink bump on lighter skin, or a brown to glossy black bump with a rolled border on darker skin. It grows slowly and rarely spreads, but it can damage surrounding tissue if left untreated.
Squamous cell carcinoma often appears as a rough, scaly patch or a sore that won’t heal. It develops on sun-exposed areas and can spread if not treated early.
Melanoma is the most dangerous form. The first sign is usually a mole that changes size, shape, or color. The ABCDE rule, developed by the National Cancer Institute, helps you spot warning signs:
- Asymmetry: one half of the mole doesn’t match the other
- Border: edges are ragged, notched, or blurred, with pigment spreading into surrounding skin
- Color: uneven shades of black, brown, and tan, possibly with areas of white, gray, red, pink, or blue
- Diameter: larger than 6 millimeters (about the size of a pencil eraser), though melanomas can be smaller
- Evolving: the mole has changed noticeably over weeks or months
Any new growth that looks like a bump, mole, or scab, any sore that won’t heal, or any existing mole that changes warrants a closer look.
How Skin Lesions Are Diagnosed
When the cause of a lesion isn’t obvious from its appearance and your medical history, several diagnostic tools can narrow things down.
A skin biopsy is the most definitive. In a punch biopsy, a small circular tool (usually 4 mm across) removes a plug of skin deep enough to reach the lower layers. A shave biopsy takes a thinner sample from the surface, while an excisional biopsy cuts out a larger or deeper section. The sample is examined under a microscope to identify the underlying cause, whether that’s infection, inflammation, or cancer.
For suspected fungal infections, a skin scraping from the edge of the lesion is treated with a chemical solution and viewed under a microscope. The presence of fungal filaments or yeast cells confirms the diagnosis. A similar scraping technique can identify scabies by revealing mites or their eggs.
A Wood light (a type of ultraviolet lamp) helps define the borders of pigmented lesions and can detect certain infections. Some bacterial infections fluoresce a characteristic bright orange-red, while specific fungal infections glow green. Patch testing is used when allergic contact dermatitis is suspected, exposing small areas of skin to common allergens to see which ones provoke a reaction. For suspected herpes infections, a Tzanck test examines cells scraped from an intact blister, looking for the distinctive giant cells that signal a herpes virus.