You can have an infectious arthritis simmering, a sinus infection, a chronic prostatitis, even a simmering endocarditis. Many of these may come from infected teeth, tooth extractions, any staph intrusion. Some people have simmering viruses in the liver, but liver blood tests would show those. Simmering Herpes simplex and Herpes zoster, and Parvo virus. My son had a mono infection (he just felt terrible and tired), for weeks before I sent him to our doctor...(when he came home from college one year) and when she did his blood work his liver enzymes were sky high. He had to wait months for his wisdom tooth extractions because of it.