A low fever is a body temperature between 37-37.8°C. A prolonged low fever is a fever that lasts for more than two weeks, excluding intermittent fevers. Many kinds of diseases can cause low fever. Fever is a symptom. Diseases such as rheumatism, tuberculosis, chronic inflammation, and immune deficiency can cause a persistent low fever. Chronic psychological stress and emotional instability can also cause a disturbance in the temperature center, resulting in an unexplained persistent low fever. Problems with any system of the body can cause a persistent low fever. The cause of low fever is difficult to detect by routine blood and urine tests. In some cases, the low fever is caused by a bacterial infection, which can only be detected by bacterial culture. Unidentified low fever is not a medicine that should be abused. Fever is one of the body’s reactions to resist disease attack, and taking antipyretic drugs in patients with low fever is not conducive to the treatment of the disease. Abuse of antipyretic drugs leads to the risk of white blood cell drop or aplastic anemia. The use of antipyretic drugs is considered only if the body temperature is above 38°C. Persistent low fever should seek early medical attention and early comprehensive examination. Some early symptoms of malignant tumors or early bacterial infections are also low fever, such as patients with lung cancer, who also have long-term low fever at the beginning.