Rashes caused by allergic reactions typically present with redness, itching, and sometimes swelling. They can occur shortly after contact with an allergen and may resolve quickly after the allergen is removed. In contrast, rashes caused by infections often come with additional symptoms like fever, pain, or pus formation, indicating an immune response to bacteria, viruses, or fungi. If a rash spreads rapidly or is accompanied by other systemic symptoms, it is crucial to seek medical advice to determine the cause.