Is It a Virus or an Allergy? How to Tell With Adult Skin Rashes

Published: 28 February 2026 · Medically reviewed content · Written for UK patients

A new rash appears on your skin and the first question is usually: what is causing this? For adults, the two most common explanations are an allergic reaction and a viral infection — but telling them apart is not always straightforward. The distinction between a skin rash allergy or virus matters, because the approach to each is quite different.

Allergic rashes are driven by the immune system's response to a specific trigger — a food, medication, environmental allergen, or contact substance. Viral rashes occur as part of the body's broader immune response to an infection. Both can produce red, itchy, or blotchy skin, which is why they are so often confused.

This guide walks through the key differences between common allergy-type and viral-type rashes in adults, offers a practical decision aid, and explains when an IgE blood test may help clarify the picture. It is written for information only — a rash should always be assessed by a qualified clinician for accurate diagnosis.

First Principles: Rashes Are Patterns, Not Diagnoses

Before looking at individual rash types, it is worth understanding a fundamental point that clinicians use in practice: a rash is a sign, not a diagnosis in itself. The same visual appearance — redness, bumps, welts — can be produced by very different underlying mechanisms.

When a GP or dermatologist assesses a rash, they consider several factors beyond what it looks like:

• Timing — How quickly did it appear? Is it related to a specific exposure, meal, or illness?
• Distribution — Where on the body is it? Is it symmetrical, localised, or widespread?
• Duration — How long do individual lesions last? Does the rash come and go?
• Associated symptoms — Is there fever, joint pain, sore throat, or any systemic illness?
• Personal history — Do you have eczema, asthma, or hay fever? Any known allergies?

The answers to these questions, combined with clinical examination, allow the clinician to narrow down the possible causes. A rash alone — even a photograph — rarely provides enough information for a definitive diagnosis (NHS, 2025).

Common Allergy-Type Rashes

Allergic rashes are mediated by the immune system and tend to have recognisable patterns. The three most common types seen in adults are urticaria, contact dermatitis, and eczema flares.

Urticaria (Hives): Fast-Onset, Move Around, Very Itchy

Urticaria — commonly known as hives — is perhaps the most distinctive allergic rash. The hallmarks include:

• Raised, pale pink or red welts (wheals) that are intensely itchy
• Individual welts that typically last less than 24 hours before fading, though new welts may appear in different locations
• A migratory pattern — the rash “moves around” the body rather than staying in one place
• A blanching quality — if you press the rash, it temporarily turns white

When comparing hives vs viral rash, the key differentiator is the transient, migratory nature of hives. Each wheal comes and goes within hours, whereas viral rashes tend to be more persistent and follow a fixed pattern.

Urticaria can be triggered by foods, medications (particularly NSAIDs and antibiotics), insect stings, latex, or environmental allergens. It can also be triggered by physical stimuli (cold, pressure, heat) or occur spontaneously without an identifiable cause. If hives last longer than six weeks, they are classified as chronic urticaria, which often has a different underlying mechanism (Allergy UK, 2025).

Contact Dermatitis: Localised to the Contact Area

Contact dermatitis produces a rash that corresponds to the area of skin that has been in direct contact with a trigger substance. The distinction between contact dermatitis vs urticaria is important:

• Allergic contact dermatitis — a delayed-type immune reaction (T-cell mediated, not IgE-mediated) that typically develops 24–72 hours after exposure. Common triggers include nickel (in jewellery and belt buckles), fragrances, preservatives in cosmetics, rubber chemicals, and hair dyes. The rash is usually red, itchy, and may blister or weep in severe cases.
• Irritant contact dermatitis — caused by direct chemical or physical damage to the skin barrier, not an immune reaction. Common culprits include detergents, solvents, and frequent hand-washing. The rash is typically dry, cracked, and sore rather than intensely itchy.

The location of the rash is the most important clue. A rash confined to the earlobes suggests nickel in earrings. A rash on the wrists or neck may point to jewellery or fragrance. A rash on the hands may indicate occupational contact or cleaning product exposure. Contact dermatitis is diagnosed through patch testing (a different process from IgE blood testing) and managed by identifying and avoiding the offending substance (NHS, 2025).

Eczema Flare: Dry, Itchy, With a Chronic Pattern

Eczema (atopic dermatitis) is a chronic skin condition characterised by dry, inflamed, itchy patches of skin. It tends to affect specific areas — the creases of the elbows and knees, the face, neck, and hands are common sites in adults. When considering an eczema flare vs allergy, the key points are:

• Eczema has a chronic relapsing pattern — it is not a single episode but a recurring condition
• Flares can be triggered by allergens (dust mites, pet dander, certain foods) but also by non-allergic factors such as dry air, irritants, infections, stress, and temperature changes
• The rash tends to be dry and rough rather than raised and migratory like hives
• There is often a personal or family history of atopy (eczema, asthma, or hay fever)

In some cases, identifying and managing allergic triggers can reduce the frequency of eczema flares. However, eczema is a multifactorial condition, and allergen avoidance alone is rarely sufficient — good skincare, emollient use, and medical management remain the cornerstones of treatment (NICE, 2024).

Common Viral-Type Rashes (Adult Considerations)

Viral rashes — sometimes called viral exanthems — are common in children but also affect adults. They differ from allergic rashes in several important ways.

Associated Fever, Flu Symptoms, and Timing After Illness

The most reliable distinguishing feature of a viral rash vs allergic rash is the clinical context. Viral rashes typically:

• Appear alongside or shortly after systemic illness symptoms — fever, fatigue, sore throat, body aches, headache, or respiratory symptoms
• Follow a predictable progression — for example, starting on the trunk and spreading outward, or appearing after fever has broken
• Persist for days to weeks rather than the minutes-to-hours cycle of urticaria
• Do not move around the body in the way hives do — the rash tends to stay in place or spread gradually

Common viral infections that can cause rashes in adults include:

• Parvovirus B19 — can cause a “slapped cheek” rash or a lace-like (reticular) rash on the trunk and limbs, sometimes with joint pain
• Epstein-Barr virus (EBV) — glandular fever can produce a widespread rash, particularly if ampicillin or amoxicillin is taken during the infection
• COVID-19 — various skin manifestations have been documented, including urticaria-like rashes, chilblain-like lesions, and morbilliform (measles-like) rashes
• Measles, rubella, and chickenpox — less common in vaccinated adults but still possible, with characteristic rash patterns

It is worth noting that some viral infections can trigger genuine urticaria — making the distinction between hives vs viral rash more challenging. In these cases, the hives are not caused by an allergen but by the immune system's response to the viral infection itself (NHS, 2025).

Drug Rashes

A frequently overlooked category is drug-related rashes, which can mimic both allergic and viral presentations. These may appear during or shortly after a course of medication (particularly antibiotics such as penicillin and amoxicillin). Some are true IgE-mediated allergic reactions, while others are non-immune pharmacological effects. Any new rash appearing during medication use should be discussed with your GP or prescriber.

A Decision Aid: Questions to Ask Yourself

While only a clinician can diagnose the cause of your rash, the following questions can help you organise your observations and provide useful information at your appointment:

What Testing Can and Cannot Do

If an allergic cause is suspected, targeted testing can help clarify the picture. However, it is equally important to understand the limitations.

IgE Blood Tests for Suspected Triggers

Specific IgE blood tests measure the level of IgE antibodies your immune system has produced against a particular allergen. They can be useful when:

• There is a clear pattern suggesting a specific trigger (e.g., hives appearing within two hours of eating a particular food)
• You want to investigate environmental allergens such as house dust mites, pet dander, or pollen in the context of eczema flares or urticaria
• You need to distinguish between true sensitisation and cross-reactivity (e.g., a peanut-positive result that may actually reflect birch pollen cross-reactivity)

Results should always be interpreted alongside your symptom history by a qualified clinician. A positive IgE result indicates sensitisation — the presence of IgE antibodies — but not necessarily clinical allergy (BSACI, 2024).

Why ‘Screening Everything’ Often Backfires

It can be tempting to request a large panel of allergen tests in the hope of identifying the cause of a persistent rash. However, broad, untargeted testing can produce more confusion than clarity:

• False positives due to cross-reactivity — IgE antibodies directed against one allergen (e.g., grass pollen) can cross-react with proteins in unrelated foods (e.g., wheat), producing a positive result that does not reflect a true food allergy
• Clinically irrelevant sensitisation — many people are sensitised to allergens they tolerate perfectly well. Discovering these incidental positives can lead to unnecessary dietary restrictions or anxiety
• Misleading reassurance from negatives — a negative IgE result does not rule out all types of allergic or hypersensitivity reaction, particularly contact dermatitis or non-IgE food reactions

BSACI and NICE guidance consistently recommends that allergy testing should be guided by clinical history — that is, testing for specific suspects rather than casting a wide net. The most informative results come from targeted testing based on a pattern of symptoms and suspected exposures (BSACI, 2024; NICE, 2024).

How Nurse-Led Testing Works at Allergy Clinic

At Allergy Clinic, we provide a diagnostic blood sampling service with nurse-led venepuncture. If you and your clinician believe that targeted IgE blood testing may help clarify whether an allergic trigger is contributing to your skin symptoms, here is how the process works:

• Choose your test: You can select from individual allergen tests, multi-allergen profiles (e.g., a food panel or environmental panel), or comprehensive screening with the ALEX² test, which covers over 300 allergens from a single blood sample. Browse available allergy tests to find the most relevant option.
• Attend your appointment: A trained nurse performs a simple venous blood draw (phlebotomy). The process is quick and straightforward — no skin exposure to allergens is involved, and you do not need to stop taking antihistamines beforehand.
• Receive your report: Your blood sample is analysed by an accredited laboratory, and you receive a detailed report showing your specific IgE results in kU/L for each allergen tested.
• Discuss with your clinician: We recommend taking your results to your GP, allergist, or dermatologist for clinical interpretation. Our service provides diagnostic data — we do not provide doctor consultations, diagnoses, or treatment plans.

This approach is designed to complement the clinical pathway, giving you and your clinician targeted information to work with.

Frequently Asked Questions

Can a viral rash look like hives?

Yes. Some viral infections can produce a rash that closely resembles urticaria, with raised, red, blotchy patches. However, viral rashes tend to appear alongside systemic symptoms such as fever, fatigue, or sore throat, and they typically persist for several days. True allergic hives tend to appear within minutes to hours of exposure to a trigger, move around the body, and individual welts usually resolve within 24 hours (NHS, 2025).

Do food allergies always cause a rash immediately?

IgE-mediated food allergies typically cause symptoms within minutes to two hours of eating the trigger food. However, the speed and type of reaction can vary. Some reactions produce urticaria rapidly, while others may cause gut symptoms or respiratory problems without a prominent rash. In rare cases, delayed reactions — such as those seen in alpha-gal syndrome — can occur several hours after eating (Allergy UK, 2025).

Can stress trigger hives?

Stress does not directly cause hives in the same way that an allergen triggers an IgE-mediated reaction. However, psychological stress is widely recognised as a factor that can worsen or prolong existing urticaria. Stress hormones can influence mast cell activity, potentially lowering the threshold for hive formation. If you notice that your rashes worsen during stressful periods, mention this to your GP as part of your assessment.

Is eczema always caused by allergy?

No. Eczema (atopic dermatitis) is a complex condition with multiple contributing factors, including genetic skin barrier dysfunction, immune dysregulation, and environmental triggers. While allergens can trigger or worsen eczema flares in some people, many eczema sufferers have no identifiable allergic trigger. IgE blood testing can be helpful when there is a clear pattern suggesting a specific allergen is worsening the eczema, but a positive IgE result does not automatically mean an allergen is driving the condition (NICE, 2024).

What photos should I take before my appointment?

Photograph the rash as soon as it appears, in good natural light, showing both close-up detail and the wider distribution on your body. Take photos at different time points to show how the rash changes. Note the date and time of each photograph. If possible, place a coin or ruler next to the rash for scale. Also photograph any swelling if present. Store photos on your phone to show your GP or specialist.

How long do allergic rashes usually last?

This depends on the type. Individual urticaria welts typically last less than 24 hours, though new welts may continue to appear. Contact dermatitis can persist for days to weeks after the trigger is removed. Eczema flares may last for weeks if not managed. If a single welt lasts longer than 24 hours or leaves bruising, this may suggest a different condition such as urticarial vasculitis, and you should seek medical review. You can read more in our guide to how long allergic reactions can last.

Should I take antihistamines before my allergy blood test?

Unlike skin prick tests, IgE blood tests are not affected by antihistamine use. You can continue taking antihistamines as normal before your blood test appointment — there is no need to stop them. This is one of the practical advantages of blood-based allergy testing over skin prick testing, particularly for people who rely on daily antihistamines for symptom management (BSACI, 2024).

Considering an Allergy Blood Test?

If your symptom pattern suggests that an allergic trigger may be contributing to your skin rash, targeted IgE blood testing can provide useful diagnostic data to share with your clinician. At Allergy Clinic, we offer nurse-led venepuncture and laboratory-analysed testing — from individual allergen tests to comprehensive panels covering over 300 allergens.

Our service provides a diagnostic blood sample and a detailed laboratory report. We recommend taking your results to your GP, dermatologist, or allergist for clinical interpretation and personalised guidance.

Explore available allergy tests and book an appointment →

Sources

• NHS — Rashes in adults, Urticaria (hives), Allergies overview, Anaphylaxis, Contact dermatitis. Available at: nhs.uk
• Allergy UK — Factsheets on skin allergy, urticaria, eczema, and food allergy. Available at: allergyuk.org
• British Society for Allergy and Clinical Immunology (BSACI) — Guidelines on allergy diagnosis, urticaria management, and testing guidance. Available at: bsaci.org
• NICE — Clinical Knowledge Summaries: Eczema, Urticaria, Contact dermatitis. Available at: nice.org.uk
• Anaphylaxis UK — Guidance on recognising and managing anaphylaxis. Available at: anaphylaxis.org.uk
• NHS inform — Viral rashes in adults, skin conditions. Available at: nhsinform.scot