My daughter, now almost 5, had a febrile seizure at a little over a year old, with a fever of 105.3. She was taken by ambulance to the hospital, blood drawn, and given tylenol. They sent us home with a vague suggestion of some mystery virus as she had no other symptoms. The fever lasted for over two months. Alternating ibuprofen and tylenol every two hours was the only way to keep her down below 100.
Every couple of months this would start and last for varying amounts of time. Finally at age 3 1/2, we saw a WONDERFUL pediatrician that suggested that we start her on Zyrtec--allergy med. He said that allergies--plural, already have her immune system working full-time and therefore overreact to any little extra thing and thus, the high fevers. After two days on Zyrtec, she was fever-free and hasn't had one since.
Every other doctor that has heard this has said that allergies don't cause fevers, and roll their eyes a bit when I tell them, but at least in this case, they did, at least indirectly, because the allergy med has stopped the fevers. So there, you doubting Docs!
I know that this may be not at all what any of you have had or have but I thought I'd throw it out just for informational purposes.