'Leaves of three, let them be.' That old advice still holds true. Poison ivy and poison oak feature three leaflets on one stem. Both plants are common causes of a red, itchy rash consisting of small bumps, blisters or swelling. The best way to prevent the rash is to avoid contact with poison ivy and other poisonous plants. If you're hiking or walking outdoors, stay on cleared paths. Wear long pants, socks and shoes to help protect your skin.