A 2-year-old baby with a low fever during the day and a high fever at night belongs to fever, which is categorized into infectious fever and non-infectious fever. If the acute onset of illness, short-term appearance of the majority of infectious fever, need to further search for the cause of fever, that is, the site of infection. The most common sites of infection that cause fever are upper respiratory tract infections and intrapulmonary infections. If the patient has symptoms such as coughing, coughing up sputum, sore throat, nasal congestion, and runny nose in addition to fever, upper respiratory tract infections are considered to be more likely. If the patient has dyspnea and chest pain in addition to fever, cough and sputum there may be a possibility of pulmonary infection. There may also be infectious diseases that are more common in children, such as measles, chicken pox and scarlet fever. In addition to fever, there will be a peripheral rash, herpes, etc. It is always recommended to observe the symptoms carefully.