A normal body temperature ranges between 96.8°F and 98.6°F. A body temperature of 99.9°F is considered a low - grade fever. Fever refers to a pathological increase in body temperature and is the most common symptom in clinical practice. It is caused by the body's response to pyrogens, which raises the set - point of the thermoregulatory center. It is an important clinical manifestation during the progression of a disease. Many infectious and non - infectious diseases can lead to fever. Generally, fever has a typical fever pattern, course, and specific clinical features. The human body has a well - developed thermoregulatory system, and the thermoregulatory center plays a key role. Since there are thermosensitive neurons in the hypothalamus, which are very sensitive to the temperature of the blood flowing through, they can quickly trigger a thermoregulatory response to maintain the balance between heat production and heat dissipation, thus keeping the body temperature relatively constant.