My Wife Has Been Having A Fever From 100-101 Degree

My wife has been having a fever from 100-101 degree fever for 7 days. She went to the doctor and he ran CBC blood test, everything was normal. Urine test came back normal as well. Ruled out flu and pneumonia. She has had some relief when she takes ibuprofen. The fever returns. Her symptoms are 1. Headaches 2. Fever. 3. Aches, 4. Lethargic.
Her doctor does not know what is causing this. He said if she got a fever again she still felt bad she should go to the ER. Can you give us any suggestions?