Fever is a common clinical symptom. High fever is defined as an axillary temperature of 39.1 to 41°C. Clinically, persistent high fever is mostly caused by infections. There are various causes of persistent high fever, firstly, infectious diseases, which can be classified as viral, bacterial, mycoplasma, chlamydia, fungal and other infectious diseases according to the pathogens. Viral infections include influenza, epidemic B encephalitis, etc.; bacterial infections include pneumonia, acute tonsillitis, sepsis, etc. Secondly, persistent hyperthermia caused by non-infectious diseases, such as intracranial injury and grand mal seizure, etc.; then there are some malignant tumors that can also cause hyperthermia, such as leukemia. When persistent hyperthermia occurs, you should go to the hospital promptly. The diagnosis can be helped by typical positive signs, such as acute tonsillitis if the pharynx is congested and the tonsils are red and swollen and overflowing with pus; if cervical tonicity is present, epidemic B encephalitis or encephalomyelitis is generally considered. If there is an increase in white blood cell count and neutrophil ratio in routine blood tests, bacterial infectious lesions can be considered. Blood culture plus drug sensitivity tests can clarify the type of infecting bacteria and guide the clinical selection of effective antibiotics. Virus isolation and viral nucleic acid testing can clarify the type of viral infection and facilitate early diagnosis. The presence of persistent high fever should be given sufficient attention, because high fever can affect the function of the heart, brain, liver, kidneys and other important organs, should promptly go to the hospital to clarify the cause of the disease and then treat the cause. Secondly, bed rest, drinking more water, physical cooling is not effective, oral antipyretic drugs such as acetaminophen or ibuprofen should be taken.