A skin rash is a visible or palpable change in the skin, and “rash” is a descriptive term rather than a specific diagnosis. A rash can involve changes in colour, texture, temperature, swelling, scaling, blistering, dryness, crusting, raised bumps or fluid-filled lesions. Because many unrelated diseases can produce similar-looking rashes, the appearance alone does not establish the cause.
Common categories include allergic reactions, irritation, infections, inflammatory skin diseases, medication reactions, heat-related disorders, insect bites and systemic diseases that affect the skin. The distribution, speed of onset, duration, symptoms, exposures and associated illness are often more diagnostically useful than the generic presence of a rash. For example, hives are typically raised and itchy and may be associated with food, pollen, insect bites or chemicals, whereas eczema usually produces dry, itchy and inflamed skin.
Allergic reactions are an important cause of rashes. In urticaria, or hives, histamine and other inflammatory mediators cause small blood vessels in the skin to become more permeable, producing raised itchy wheals that can appear and disappear relatively quickly. Allergic contact dermatitis is different: the immune system reacts to a substance that has previously sensitised it, such as nickel, fragrance, hair dye, preservatives, latex or particular topical medicines.
The resulting rash commonly affects areas that have contacted the allergen and may become red, itchy, swollen, cracked, blistered or weepy. Some allergic reactions can be systemic and potentially dangerous, particularly when rash is accompanied by swelling of the lips or tongue, breathing difficulty, wheezing, faintness or circulatory problems.
Irritation can produce a rash without requiring an allergy. Irritant contact dermatitis occurs when a substance directly damages the skin barrier, particularly after repeated exposure to relatively mild irritants such as soaps, detergents or water, or after shorter exposure to stronger chemicals. Solvents, disinfectants, acids, alkalis, cement, oils, powders and some plants can also irritate skin.
Repeated washing and wet work can be particularly important because they remove lipids from the outer skin barrier and increase water loss, leaving skin more susceptible to inflammation. Heat, cold, friction and low humidity can intensify symptoms once irritation has developed. This distinction between irritation and allergy matters because irritant dermatitis can affect anyone exposed sufficiently to a damaging substance, whereas allergic contact dermatitis involves an immune sensitisation process.
Infections can cause rashes through direct effects of microorganisms, immune responses to infection or both. Viral illnesses can produce widespread maculopapular rashes, vesicles or other patterns; examples include chickenpox, measles and shingles. Bacterial infections can produce localised inflammation, crusting, pustules or rapidly spreading redness, while fungal infections commonly produce scaly or ring-shaped lesions. Parasitic infections such as scabies can cause intensely itchy rashes, often with characteristic distribution and household spread.
Some rashes associated with infection are particularly important because they can indicate serious systemic disease. A rapidly spreading rash accompanied by fever, severe illness, confusion or other systemic symptoms requires urgent assessment rather than being assumed to be a minor skin condition. The cause can often only be established by combining the skin findings with the person's history and examination.
Inflammatory skin diseases are another major category. Atopic eczema results from interactions among genetic susceptibility, impaired skin-barrier function, immune dysregulation and environmental influences. Psoriasis involves abnormal immune signalling and accelerated epidermal growth, producing characteristic plaques with scale. Rosacea causes persistent or episodic facial flushing, redness and sometimes papules and pustules. Vitiligo causes loss of melanocytes or melanocyte function and therefore produces depigmented areas rather than a conventional inflammatory rash. Autoimmune diseases such as lupus can also cause characteristic skin manifestations. These conditions may fluctuate over time and can have specific triggers or associations, so the word “rash” should not be treated as synonymous with infection or allergy.
Medicines can cause several different types of skin eruption. A drug may produce an uncomplicated exanthem, photosensitivity, urticaria, or a more serious immune-mediated reaction. Antibiotics, anticonvulsants, anti-inflammatory medicines and many other drugs can be responsible in particular circumstances. Some medicines also produce skin changes indirectly by altering immune function or increasing sensitivity to ultraviolet light. The timing relative to starting or changing medication is therefore diagnostically important, although reactions can occasionally occur after prolonged exposure.
A new rash after a medicine should be assessed in context rather than automatically attributed to the drug, because infections and unrelated skin diseases can occur at the same time. Blistering, skin peeling, painful widespread redness, mucosal involvement, facial swelling, breathing difficulty or systemic illness following a medicine are warning signs requiring urgent medical attention.
Physical and environmental factors can also cause rashes. Heat rash, or miliaria, occurs when sweat ducts become blocked and produces small itchy or prickly lesions, particularly in hot and humid conditions. Sun exposure can cause sunburn or trigger conditions such as polymorphic light eruption, which produces itchy or burning lesions after ultraviolet exposure. Cold, friction, pressure, sweating and changes in humidity can aggravate several inflammatory skin conditions.
Insect bites can create local inflammatory reactions ranging from small itchy papules to substantial swelling, while repeated scratching can damage the skin and cause secondary infection. The timing of a rash relative to heat, sunlight, exercise, clothing, occupational exposure or outdoor activity can therefore provide useful diagnostic evidence.
The body's wider medical state can sometimes explain a rash. Liver, kidney, thyroid, autoimmune, haematological and connective-tissue disorders can produce cutaneous manifestations. Nutritional deficiencies can cause skin changes, while hormonal changes can influence conditions such as acne and eczema. Stress can worsen some inflammatory disorders without being a universal primary cause. Pregnancy can alter immune and hormonal physiology and can produce or modify several skin disorders.
Because the skin is an organ connected to the immune, vascular, endocrine and nervous systems, a rash can occasionally be an external manifestation of internal disease. The diagnostic process therefore considers age, medical history, medications, allergies, recent infections, travel, occupation, household contacts, new products and foods, and the exact chronology of the eruption.
