What Is The Treatment For Night Sweats , Fever And Headache?

My husband, 55, very healthy and active (one of those never been sick kind of guys) came down with extreme chills suddely 4 weeks ago, followed by nights sweats for weeks, high fevers, heaaches, exhaustion and now abdominal pain and frequent bowel movements. He has had blood work, flu test, antibiotics for possible tick bite, chest xray...What could this be?