Skin diseases arise from a wide range of causes, including infections, genetics, immune system malfunction, environmental exposure, chemical irritants, underlying health conditions, and psychological stress. Most skin conditions involve more than one of these factors working together. Dermatitis (eczema) alone affects roughly 1 in 18 people worldwide, making skin disease one of the most common health concerns globally.
Bacterial and Viral Infections
The two most common bacterial skin pathogens are Staphylococcus aureus (staph) and group A streptococci (strep). Staph bacteria cause a range of conditions depending on how deep the infection goes. At the surface level, staph triggers folliculitis, which appears as tiny red pustules around hair follicles. Deeper staph infections produce boils, where the surrounding tissue becomes swollen and painful. Staph also causes a serious condition called staphylococcal scalded skin syndrome, in which a toxin produced by the bacteria leads to widespread reddening and peeling of the skin.
Strep bacteria are the primary cause of cellulitis, a spreading infection of the tissue just beneath the skin that causes redness, warmth, and swelling. Both staph and strep can cause impetigo, the crusty, weeping sores common in children.
Viruses cause their own set of skin conditions. Human papillomavirus produces warts. The varicella-zoster virus causes chickenpox on first exposure and can reactivate decades later as shingles. Herpes simplex viruses cause cold sores and genital herpes. Fungal organisms are responsible for ringworm, athlete’s foot, and yeast-related conditions like thrush and tinea versicolor.
Genetic Mutations
Some skin diseases are caused by inherited mutations in genes that build the structural proteins holding skin together. The clearest examples are the epidermolysis bullosa (EB) family of diseases, where the skin blisters from minimal friction or pressure.
In the simplest form, epidermolysis bullosa simplex, point mutations in keratin genes cause the structural proteins in the outermost skin layer to break down under stress, leading to blistering. In junctional EB, mutations affect a different protein (laminin) that anchors the outer skin layer to the tissue beneath it. In dystrophic EB, the most severe form, mutations in the gene for type VII collagen destroy the anchoring fibrils that connect the deeper skin layers, causing the skin to separate with even gentle contact.
Other genetic skin conditions include harlequin ichthyosis, caused by mutations in a transporter protein essential for forming the skin’s outer barrier. Without it, the skin develops thick, diamond-shaped plates separated by deep cracks. These conditions are rare individually, but collectively, hundreds of single-gene skin diseases have been identified.
Immune System Malfunction
In autoimmune skin diseases, the immune system attacks healthy skin cells. Psoriasis is the most well-known example, affecting about 354 out of every 100,000 people worldwide. In psoriasis, specific immune cells (T cells) become overactive and release inflammatory signaling molecules, particularly interleukin-17 and tumor necrosis factor-alpha. These signals accelerate skin cell production dramatically, causing the thick, scaly plaques characteristic of the disease.
Vitiligo results from the immune system selectively destroying melanocytes, the cells that produce skin pigment. The exact trigger remains unclear, but oxidative stress, metabolic abnormalities, and inflammation all appear to contribute. The result is expanding patches of depigmented skin.
Dermatitis, or eczema, carries the heaviest burden among immune-related skin conditions, with a global prevalence rate of about 5,459 per 100,000 people. It involves a dysfunctional skin barrier combined with an overactive immune response that produces chronic itching, redness, and inflammation.
Allergens and Chemical Irritants
Contact dermatitis is one of the most common skin diseases, and it comes in two forms. Allergic contact dermatitis happens when your immune system develops a reaction to a specific substance. The most frequent triggers are nickel (found in jewelry, belt buckles, and phone cases), fragrances in cosmetics and skincare, preservatives, botanical ingredients, and medications applied to the skin including some antibiotic creams.
Irritant contact dermatitis doesn’t involve an immune reaction. Instead, a substance directly damages the skin barrier. Common culprits include detergents, soaps, cleaning products, acids, solvents like nail polish remover, paints and varnishes, hair dyes, and epoxy resins. Repeated exposure to even mild irritants, like frequent handwashing with soap, can break down the skin’s protective layer over time and produce a chronic rash.
Environmental and UV Damage
Air pollution damages skin through several pathways. Particulate matter, ground-level ozone, nitrogen dioxide, sulfur dioxide, and volatile organic compounds all generate oxidative stress in skin cells. Some pollutants, particularly polycyclic aromatic hydrocarbons found in vehicle exhaust and industrial emissions, become more damaging when activated by ultraviolet radiation. This means the combination of sun exposure and pollution is worse for your skin than either factor alone.
UV radiation on its own is a major cause of skin damage. It directly harms DNA in skin cells, breaks down collagen, suppresses local immune function, and triggers inflammation. Over time, these effects contribute to photoaging, precancerous growths, and skin cancer. Even a single severe sunburn causes measurable DNA damage that the body may not fully repair.
Underlying Health Conditions
Internal diseases frequently show up on the skin first. Diabetes is linked to several skin conditions, including necrobiosis lipoidica, a chronic inflammatory condition that causes waxy, yellowish-brown patches typically on the shins. People with diabetes are also more prone to bacterial and fungal skin infections because elevated blood sugar impairs immune function and feeds microbial growth.
Inflammatory bowel diseases like Crohn’s disease and ulcerative colitis are associated with pyoderma gangrenosum, a condition where painful ulcers develop on the skin, often on the legs. Chronic liver disease, including hepatitis, can cause jaundice, spider-like blood vessel clusters on the skin, and intense itching. Thyroid disorders commonly trigger dry, coarse skin or, in the case of an overactive thyroid, warm and unusually smooth skin with increased sweating.
How Stress Gets Under Your Skin
Psychological stress triggers a measurable biological cascade that directly affects skin. When you’re stressed, your brain activates the hypothalamic-pituitary-adrenal (HPA) axis, flooding your body with cortisol. Elevated cortisol suppresses certain immune functions while amplifying others, shifting the immune system toward a more inflammation-prone state. But the connection goes deeper than that: your skin has its own miniature version of this stress-response system, producing the same stress hormones locally.
Stress also causes nerve endings in the skin to release a neuropeptide called substance P, which triggers mast cells to release histamine and other inflammatory molecules. Mast cells act as a central hub for stress-driven skin inflammation. They respond to signals from the brain’s stress pathways, the local skin stress system, and the nervous system simultaneously. The result is increased blood vessel permeability, immune cell infiltration, and a flood of inflammatory signals.
This is why stress reliably worsens psoriasis, eczema, acne, contact dermatitis, and alopecia areata. The effect isn’t imagined or indirect. Stress hormones and neuropeptides physically alter immune activity and inflammation in skin tissue.
How Skin Diseases Are Diagnosed
When a skin condition isn’t identifiable through a visual exam and medical history alone, doctors use several targeted tests. Patch testing involves placing small amounts of common allergens on the skin under adhesive patches for 48 hours to identify what’s triggering an allergic reaction. Skin biopsies, where a small sample of tissue is removed for microscopic analysis, come in three types: punch biopsies use a circular blade to take a core sample, shave biopsies remove a thin surface layer, and wedge excisions cut out a larger section for deeper analysis.
Scrapings collect surface cells or scales to check for fungal infections under a microscope. A Wood light, which emits ultraviolet light, can reveal fungal infections and pigment disorders that aren’t visible under normal lighting. Tzanck testing examines cells from the base of a blister to identify viral infections like herpes. These tools help pinpoint the specific cause when multiple possibilities could explain the same set of symptoms.