Most rashes are harmless and caused by viruses, allergies, eczema, or skin irritation. A few, however, are signs of serious illness and need emergency care. A rash of small red or purple spots or blotches that does not fade when you press a clear glass against it can be a sign of meningitis or blood poisoning and is a medical emergency, especially with fever, headache, stiff neck, or drowsiness.
Also seek urgent care if a rash appears suddenly with swelling of the face, lips, or throat, difficulty breathing, or dizziness, as this may be anaphylaxis. Widespread blistering, skin that peels or sheds, painful sores in the mouth or eyes, or a rash with high fever that started soon after a new medication can indicate a severe drug or skin reaction and need immediate assessment.
For itchy but otherwise well rashes, a pharmacist or telehealth review is often enough. When a rash is accompanied by any of the warning features above, treat it as urgent and seek in-person care.