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A fever, or pyrexia, occurs when body temperature rises above 100.4 degrees Fahrenheit (38°C). A person with fever is termed febrile. Fever is a natural defense mechanism that activates the immune system to fight infections caused by pathogens such as bacteria and viruses.

The hypothalamus regulates core body temperature through a thermoregulatory setpoint. When immune cells detect pyrogens from pathogens, they release inflammatory cytokines like interleukin-1 and tumor necrosis factor. These cytokines stimulate the hypothalamus to produce prostaglandin-E2, which raises the temperature setpoint and triggers the body to produce heat.

During onset, pyrogens from pathogens activate immune cells like macrophages and lymphocytes, which produce inflammatory cytokines. These cytokines prompt the hypothalamus to produce prostaglandin E2, adjusting the setpoint upward. The body perceives itself as colder than it actually is, triggering heat-production mechanisms that may take several hours to raise temperature to the new setpoint.

During the course phase, as body temperature rises to the elevated setpoint, patients experience chills, shivering, body aches, fatigue, and loss of appetite. These symptoms occur because the body feels cold despite the actual temperature increase. This phase ends when the body reaches the higher setpoint, causing sensations of warmth and cessation of chills.

During resolution, the body eliminates the pathogen and lowers the hypothalamic setpoint. Heat-loss responses such as vasodilation and diaphoresis (sweating) occur, aiding temperature reduction. Once fever resolves, the patient is termed afebrile. Fever also activates interferon, a natural antiviral agent that helps combat infection.

Hyperthermia is increased body temperature due to prolonged exposure to extreme heat, not from fever-causing pathogens. Unlike fever, hyperthermia doesn't respond to antibiotics because the thermoregulatory mechanism is overwhelmed and fails to balance excessive heat. Immediate cooling treatment is vital, especially in conditions like malignant hyperthermia triggered by certain anesthetics.

Temperatures exceeding 40°C (104°F) can lead to seizures, confusion, or irreversible brain damage and require urgent intervention. Healthcare professionals assess fever using guidelines for measuring vital signs and various equipments used to measure body temperature at different sites to ensure accurate monitoring and appropriate patient care.