The few possibilities which I can think of are
1. Infectious mononucleosis
It occurs most commonly in adolescents and young adults, where it is characterized by fever, sore throat, muscle soreness, and fatigue. Infectious mononucleosis typically produces a mild illness and is often asymptomatic. Mononucleosis is predominantly caused by the Epstein-Barr virus (EBV), which infects B cells (B-lymphocytes), producing a reactive lymphocytosis predominantly consisting of atypical lymphocytes, a specific type of T-cell that gives the disease its name.
Laboratory findings usually include an elevated white blood cell count and abnormal liver function tests. The white cell count elevation is predominantly in the lymphocyte portion, and of those the majority is often of the atypical form characteristic of the disease.
2. Enteric Fever (Typhoid fever) – Although the first blood culture is negative in your case, still a repeat blood culture may help.
3. Chronic Liver Disease – I’ll request you to please get a complete liver function tests done to rule the possibility of the same.
4. If you have recently visited a tropical country, then there’s a chance of tuberculosis too which presents as a low grade fever, loss of appetite and maybe with raised ALT. For that, you may need an Xray of the chest, ESR,tuberculin test etc.
You need a thorough examination and complete investigations as the doctors treat such patients as FUO (Fever of unknown origin).
I hope my endeavor has helped you.
Please do keep me posted.
It is difficult to give any precise answers to questions with such open dimensions.
Low grade fever may be caused by many reasons.
In this case this may be due to some infection in the intestinal or hepatobilary system. It may also be due to some changes in the intestinal flora or fauna.
I would like to get a more detailed history before I can be comfortable in answering this question. This must also include, drug, smoking, alcohol histories.
Let me know if you may have some other questions