Recurring low grade fever in an infant

My granddaughter keeps having a low grade fever. Ist time she was 5 weeks fever reached 102. spent 2 days on antibiotics in the hospital they ran all kinds of tests (and a spinal tap) with no answer, the fever went away.  At 8 weeks it came back she spent another 3 days in hospital (on antibiotics) with no answers. They keep saying it's viral since no cultures come back positive.  Now she is 15 weeks and it's back.  She has been running a fever from 99.5 to 100.8 for 6 days straight.  Went to the Dr. on Tues (this is Sunday) and there was nothing wrong again.  She acts fine, she's growing well.  We are really getting concerned because no Dr. seems to be able to come up with any answers.  Do you have any ideas?

Thank you,
concerned Grammy  
To:

[Deanna L Aftab Guy, MD]

Your question is not really an endocrine problem, it will be best directed to an infectious disease forum or pediatrics forum, thanks!