The causes of low fever and fatigue for three months may be tuberculosis, tumor, functional low fever, etc., which need to be analyzed specifically. 1. Tuberculosis: low fever and fatigue will appear after tuberculosis infection, which may be accompanied by respiratory symptoms such as cough, sputum, bloody sputum, etc. Tuberculin test, chest X-ray and other examinations can assist in definite diagnosis. 2. Tumor: Tumor cells can secrete pyrogens, which can stimulate the thermoregulatory center, resulting in low fever, fatigue, accompanied by symptoms such as emaciation, and tumor markers, PET-CT and other tests can assist in making a clear diagnosis. 3. Functional hypothermia: hypothermia and fatigue can occur when there is dysfunction of body temperature regulation center, but there is no other organic lesion, so the diagnosis needs to exclude other organic lesions. There are many reasons for low fever and fatigue for three months, so it is recommended to consult a doctor in time to avoid delaying the condition.