I've heard of getting hives after a high fever. My niece had a really bad fever with an virus late last year. About a week or so after getting over it, that is without having a fever or any other symptoms, she suddenly flared up in this hideous rash head to to toe. A very speedy trip to the hospital confirmed that it was a reaction to the fever and that it does happen. She was put onto antihistamines to help clear it up.