Red, round patches on the arm can be worrying, especially when they appear in clusters or slowly expand. Whether they’re contagious depends on the cause: some circular rashes spread through contact, while others don’t spread at all. Figuring out which one you’re dealing with matters for both treatment and prevention.

A common contagious cause is ringworm, also called tinea corporis. It’s a fungal infection (not a worm) that thrives in warm, moist areas and often shows up as red, round or oval patches with slightly raised or scaly edges, a clearer center, and mild to moderate itching. It spreads through skin-to-skin contact, shared towels or clothing, gym equipment, infected pets, or contaminated surfaces, and it may slowly grow or form new nearby circles.

Not all circular rashes are infectious. eczema (nummular dermatitis) can cause coin-shaped, itchy red patches due to skin barrier problems and inflammation, not infection. psoriasis causes red plaques with thicker, silvery scales from immune system overactivity. Neither spreads from person to person.

Looks can offer clues but aren’t foolproof. Fungal rashes tend to have a sharp outer border and expand outward; eczema often feels very dry and can ooze or crust when severe; psoriasis plaques are thicker and commonly appear on elbows, knees, or the scalp. If a rash worsens with steroid creams, that can hint at a fungal cause, since steroids may let fungi spread.

Get medical advice if the rash keeps spreading, becomes painful or intensely itchy, or doesn’t improve with over-the-counter treatments, or if you’re unsure what it is. A clinician can do a simple skin scraping to confirm a fungal infection when needed.

If a fungal infection is suspected, keep the area clean and dry, avoid scratching, don’t share towels or clothing, wash bedding regularly, and treat pets if a vet confirms infection. Prompt antifungal treatment usually clears ringworm within a few weeks.