Hi, I'm new to this site, but I saw this post and I just had to reply to it. My son is 18 mo now, and about 6 mo ago he just broke out in hives - and it looked really bad, and kept getting worse. I gave him benadryl, and it did let up some. They would come and go, and they were mostly on his torso, but some on his arms and legs and face. At first I thought it was a food allergy, but then after eliminating all possible causes, he got worse! He didn't really run a fever, and the rash didn't look like anything like fifths, or Hand Foot, Mouth or any of those childhood rashes.
I ended up taking him to my pediatrician (whom I really trust) and he said it was definately hives (ideopathic urticaria is the technical name) and could be viral. He said I didn't have to give him anything for it it would clear up on its own with in 6 weeks. But he gave him some Zyrtec (a prescription antihistamine that lasts for 24 hrs) just to help with the visible symptoms. The hives ended up lasting for 6 weeks, then they just went away.
Throughout all this the zyrtec would help a little, but never totally cleared it up. So what happens is a virus enters the body, and for some reason the body has an allergic reaction to the actual virus. It's kind of rare, but can happen. Hope this helps.