Low fever, headache and lack of energy may be caused by upper respiratory tract infection, lupus erythematosus, tuberculosis, rheumatic fever, tumor and other diseases. 1. Upper respiratory tract infection: this disease is mainly caused by the infection of pathogenic microorganisms such as parainfluenza virus, adenovirus, staphylococcus, streptococcus pneumoniae and so on. It can lead to low-grade fever and headache, as well as symptoms such as fatigue and limb pain, which are mostly manifested as lack of energy. 2. Lupus erythematosus: The cause of this disease is still unclear, and it may be related to heredity, estrogen abnormality, pathogenic microbial infection, sunlight and other factors. Symptoms are atypical in the early stage, but some patients may have symptoms such as low-grade fever, headache and lack of energy. 3. Tuberculosis: The occurrence of this disease is related to Mycobacterium tuberculosis infection, which can be triggered by factors such as low immunity. Some patients may have persistent or intermittent afternoon low-grade fever, headache, fatigue, etc. Common symptoms include coughing, coughing up sputum, and blood in sputum. 4. Rheumatic fever: Group A beta-hemolytic streptococcal infection is the main cause of rheumatic fever. Typical symptoms are wandering polyarthritis, subcutaneous nodules, erythema annulare, etc., which can be accompanied by fever, sore throat, cough and other symptoms, and some patients will also have low-grade fever, headache and lack of energy. 5. Tumor: leukemia, lymphoma, etc. can be manifested as low-grade fever, headache and lack of energy, the cause is unknown. There are many reasons for low fever, headache and lack of energy, you need to go to the hospital to do the relevant examination to clarify the specific causes of the disease.