@turkay1-- Unexplained fevers in adults can be due to Lyme disease, I know because my cousin had it.
Lyme disease is caused by a tick bite and barely causes any symptoms in the beginning. If someone doesn't notice the bite, there won't be symptoms until weeks or months later and the first symptom is usually an unexplained fever.
After fever, muscle aches and pains, chills, neck and head pain starts.
If it's not treated Lyme disease can also cause meningitis and chronic muscle inflammation and pain. My cousin's was treated very early on so she didn't develop these complications.