Brother has mysterious virus and drs can’t figure out the issue

Hi, I’m posting this on behalf of my brother who has been very ill with no answer as to why He is male, 27, roughly 170lbs. No medical issues. No smoking or drinking. In good shape (exercises regularly-boxing). No medications. A week and a half ago my brother came down with a flu. No coughing or runny nose or sore throat pain. But he was experiencing a high fever, nausea, headaches and chills. About 4 days went by and he wasn’t getting better. Took him to the ER as he was advised by the dr at the clinic that if he didn’t get better there was a possibility of sepsis. Went to the ER and they ran many tests- blood, urinalysis, CT scan, ultra sound. Checked his lungs and brain and did everything to see if he might have had some type of infection. Nothing. All clear. They kept him in the hospital for 3 days with IV hydration and Tylenol. They were concerned about his fever as it wasn’t getting any better. But the fever has not gone away. It’s been a week and a half. He is experiencing migraines. And the on and off chills and sweats. He is so weak. The fever is now down to occurring at night as opposed to on and off all throughout the day. But overall he’s not doing much better. He can barely walk. What might this virus be? The drs said it’s not an infection but most likely a virus. But if it was your typical flu wouldn’t he be better by now? His body is having a really hard time fighting this thing off.