I don't know how high the fever is but it could be roseola. Think I spelled that right. My baby had it before she was 1. Several other babies I know have had it. The illness mainly starts as a sudden high fever. I put my daughter down for nap and she was fine then when she woke she had like 102 fever. There are no real symptoms other than fever. No vomiting no diahrea nothing. Just fever. So we had to take her to dr. and have finger prick test. No infection showed and then fever still wouldn't go down after another day and higher fever we went to the ER they drew blood (it was awful) and urine test. No infection - they say it is a virus and go home. We wait a few more days and go back to dr. That day the rash showed up. That is roseola - high fever - lack of appetite and no other symptom then in 4 -8 days (for us it was 8 of fever) a red splotchy rash shows up especially on the trunk area. That is when you know what they had. So infuriating to watch your baby sick and not know what is wrong until the illness is finally over. When the rash shows up the fever usually goes down and then they get well.
I don't know if that is what your baby has but it could be since you are not seeing any other symptoms.
Hope that helps.