Rashes are small, often-red bumps that appear on the skin; hives are raised white or yellow bumps surrounded by red, inflamed skin. Both rashes and hives usually start as a burning sensation followed by itching. They can be caused by an allergen or irritant from food, clothing or household chemicals. Medications, poor digestion or illness may also cause rashes or hives. One of the most common forms of a rash is contact dermatitis, which is caused when a substance of some sort irritates the skin. A more severe reaction may cause swelling, redness and larger blisters. Rashes can be caused by many things, including poison ivy, poison oak, soaps, detergents, shampoos, perfumes, lotions, a latex allergy and a food allergy. They may also occur from viral infections such as shingles, fungal infections, acne, eczema, psoriasis, rosacea, Lyme disease, bug bites or heat exposure.