Low fever for half a month can be caused by tuberculosis, tumor, rheumatic fever, hyperthyroidism, etc. It is recommended to consult a doctor in time. 1. Tuberculosis: it is a chronic infectious disease triggered by Mycobacterium tuberculosis infection. Long-term low fever is the most common systemic symptom of the disease, and patients can also have fatigue, night sweats, loss of appetite, weight loss and other discomforts, as well as other related symptoms when infected with other systems, for example, patients with pulmonary tuberculosis can also have coughing, coughing up sputum, hemoptysis, chest pain and other discomforts. 2. Tumor: prolonged low fever is mostly seen in patients with malignant tumors, and the patients may also have systemic symptoms such as anemia, lethargy, fatigue, etc. When the tumors are large, they may also have symptoms such as local compression, bleeding, pain, obstruction and other related symptoms. 3. Rheumatic fever: it is caused by group A type B hemolytic streptococcus infection, and there may be precursor symptoms such as low fever, sore throat, cough, submandibular lymph node enlargement, etc., 1-6 weeks before the appearance of typical symptoms, and after that, there may be arthritis, cardiac inflammation, subcutaneous nodules, annular erythema, chorea, etc. Typical symptoms may appear. 4. Hyperthyroidism: Patients may have high metabolic symptoms such as low-grade fever, fatigue, excessive sweating and weight loss due to increased secretion of thyroid hormone, in addition to other symptoms such as anxiety, agitation, palpitation, hyperphagia and flushing of the face. Patients with symptoms of low-grade fever, which lasts for half a month, should consult a doctor in a timely manner to identify the cause of the disease under the guidance of the doctor and carry out the appropriate treatment, and should not make blind judgments on their own, so as not to delay the condition.