Almost everyone has had a fever, with different symptoms and various causes, but what exactly is considered a fever? Normal human body temperature is regulated by the thermoregulatory center, and heat production and heat loss are in a dynamic balance, maintaining human body temperature within a relatively constant range. The normal oral temperature range is 36.3℃-37.2℃, the rectal temperature is generally about 0.3℃-0.5℃ higher than that of the oral cavity, and the axillary temperature is generally 0.2℃-0.4℃ lower than that of the oral cavity. In the physiological state, different individuals, the same body at different times and different environments, their body temperature will vary: 1, different individuals: children due to high metabolic rate, body temperature can be higher than adults; elderly metabolic rate is low, body temperature can be lower than adults, the basal body temperature of individuals can be slightly higher or slightly lower than the normal range of 0.5 ℃. 2, the same body at different times: under normal circumstances, the human body temperature is lower in the morning and higher in the afternoon, but generally fluctuate within no more than 1 ℃; women’s body temperature is higher during ovulation and pregnancy, and lower during menstruation. 3. Different environments: body temperature is higher during exercise, meals, emotional excitement and working in a hot environment, and lower in a cold environment. In pathological conditions, when the body temperature rises above the normal range due to increased heat production or (and) decreased heat dissipation for various reasons, it is called fever. A fever is a fever when the thermoregulatory center is affected by a thermogenic agent or a dysfunction of the thermoregulatory center, which increases heat production and decreases heat dissipation, resulting in an increase in body temperature above the normal range. Generally speaking, if the oral temperature is higher than 37.3℃, or the anal temperature is higher than 37.6℃, or the body temperature changes more than 1.2℃ in a day, it can be considered as having fever. Fever classification: 1, low fever: oral temperature > 37.3 ℃, anal temperature > 38.0 ℃, body temperature change between one day > 1.0 ~ 1.2 ℃; 2, moderate fever: 38.1 ℃ ~ 39.0 ℃; 3, high fever: 39.1 ℃ ~ 41.0 ℃; 4, ultra-high fever: > 41 ℃. According to the fever range fever is divided into: 1, acute fever: fever range within 2-3 weeks; 2, long-term fever: body temperature > 38 ℃, fever range > 2-3 weeks; 3, long-term low fever: body temperature in 37 ~ 38 ℃, fever range > 4 weeks. There are many causes of fever, which are divided into two categories: infectious fever and non-infectious fever according to the presence or absence of pathogens invading the body. If fever occurs, especially long-term fever, you should go to the hospital promptly to identify the cause.