Why does cellulitis cause spreading redness?

Most people with Cellulitis notice spreading redness. Bacteria can enter through a cut, crack, bite, ulcer, or another break in the skin. The immune response widens around the infected area, increasing blood flow and causing warmth, swelling, tenderness, and a red or darker-looking patch. The visible edge may enlarge as inflammation reaches adjacent tissue, although colour can be harder to judge on darker skin. A clinician may examine the border, temperature, pain, swelling, and general symptoms to distinguish cellulitis from irritation, allergy, or other causes.

Arrange prompt medical assessment for new redness that is warm, painful, swollen, or getting larger. Keep the area clean, avoid squeezing or scratching it, and rest the affected limb if movement increases discomfort. Do not use leftover antibiotics or stop a prescribed course without medical advice. A clinician may recommend treatment based on the site, severity, allergies, and other health conditions. Marking the outer edge with a pen can help you describe whether it is changing, but it does not replace examination.

Seek urgent help if redness is rapidly expanding, pain is severe or out of proportion, the skin becomes dusky or blistered, or you feel faint, confused, very unwell, or short of breath. Fever, chills, red streaks, pus, or swelling near the eye also need prompt evaluation. People with diabetes, poor circulation, a weakened immune system, or a recent operation should seek advice early because infection can progress more quickly.

Ask what may have allowed bacteria to enter, whether the redness should be measured or reviewed, which treatment is appropriate, and what changes should prompt immediate reassessment.