I've had 7 months of fever (I'm only 28) and they finally found I had ongoing mono. Get you EBV levels checked to see if this is the cause. Another possibility is Chronic Fatigue Syndrome. Some doctors don't believe it exists, but the number of believers is growing. Also look into lyme disease, celiac sprue, irritable bowel syndrome, autoimmune diseases (get your ANA levels checked), check your vitamin D levels (mine were extremely low), get hormone levels checked as well (make sure you're not going through early menopause, my OBGYN thought this was the case for me at first), and check your thyroid levels. Also, if you have good insurance, get a CAT scan of your sinuses to make sure you don't have a sinus infection - they can be hard to detect and often don't affect blood work. Finally, I would also see an allergiest to make sure you haven't developed an allergy.
The most imporant thing I can tell you is keep seeing doctors until you find one who will listen. It's helpful if you can document your symptoms. Keep a chart of when you feel badly, what you ate that day, and bring it in so they can review it.
I had same, undiagnosed symptoms for *EVER* and it turned out I was horribly anemic. Once the Hemoglobin levels were brought up and maintained the low-grade fever has disappeared. It got to the point where I figured I just regularly ran a temp. of 99.5...like my son's "normal" is 97.8...anyway...fatigue continues and I've been chasing a diagnosis for over a year. But the long-term, low-grade fever was resolved with iron infusions and b12 shots.
I went through about two years of unexplained low-grade fevers and flu-like symptoms... I would go from feeling fine to absolutely miserable and back within a few minutes. I had chest x-rays, chest/ab/pelvic CTs, lots of blood tests and even an Indium white cell scan - all were negative. In knew I was in trouble when my primary care physician: a brilliant specialist in infectious diseases couldn't figure it out. I was referred to a bunch of other doctors, including the Johns Hopkins division of Infectious Diseases to no avail... no one could figure out what was wrong and I could tell that everyone was starting act like it was all in my head. Well, it was... it turned out to be a chronic sinus infection that showed few external signs, I only figured it out because I kept getting this horrible mothball breath that would come and go, generally getting worse when my symptoms got worse.
Crhanse made some great suggestions and is absolutely right... sinus infections can be EXTREMELY hard to detect and won't necessarily affect blood work. I had no idea that a sinus infection could prove to be a hidden source of so much misery.
I understand how frustrating this can be... good luck!
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