A body temperature above 38°C (100.4 °F) is known as fever or pyrexia, and a person with fever is termed 'febrile.' Typically, the hypothalamus, a part of the brain that acts as the body's thermostat, regulates body temperature through a thermoregulatory setpoint. It receives signals from cold and warm thermal receptors throughout the body and adjusts the body's temperature accordingly. Fever occurs when this hypothalamic setpoint is altered, usually in response to an infection or illness.

Grasping the three phases of fever - onset, course, and resolution - is not just informative, but empowering. It offers a comprehensive insight into the body's response to pathogens, equipping healthcare professionals and medical students with the knowledge to provide superior patient care.

Onset Phase: The onset phase begins with the initial exposure to a pathogen or trigger for fever. This could be a virus like influenza, a bacterium like streptococcus, or other foreign substance that enters the body. Pyrogens, such as bacteria and viruses, raise body temperature by acting as antigens to activate the immune system. Immune cells like macrophages and lymphocytes detect pathogens and respond by producing inflammatory cytokines (TNF, IL-1, IL-6). These cytokines prompt the hypothalamus to produce prostaglandin E2 (PGE2), which adjusts the hypothalamic setpoint upwards, making the body perceive it as colder than it is. This misperception triggers mechanisms to raise the body's temperature to the new setpoint, which may take several hours.

Course Phase: The body's temperature rises once the new setpoint is established. During this phase, symptoms such as chills (shivering or feeling cold), body aches, fatigue, and loss of appetite, occur, as the body feels cold despite the actual increase in temperature. These symptoms are the body's response to the elevated temperature and the immune response to the pathogen. This phase ends when the body reaches the higher temperature setpoint, leading to a sensation of warmth and cessation of chills.

Resolution Phase: The body eventually eliminates the pathogen and strives to restore balance. The fever breaks, and the body's temperature returns to normal. The hypothalamic setpoint is lowered, leading to heat-loss responses such as vasodilation and diaphoresis (sweating), aiding in temperature reduction. Once the fever resolves, the patient is termed 'afebrile.' While fever often subsides on its own, it's crucial to be vigilant and prevent temperatures exceeding 40°C (104°F), which can lead to seizures, confusion, or irreversible brain damage. Fever also activates interferon, a natural antiviral agent within the body.

Hyperthermia: It's crucial to distinguish this condition, which is characterized by an increased body temperature due to prolonged exposure to extreme heat, from fever. Unlike fever, hyperthermia doesn't involve fever-causing pathogens and, therefore, doesn't respond to antibiotics. It's a result of an overwhelmed thermoregulatory system that can't compensate for excessive heat. Immediate treatment through cooling methods is vital. Conditions like malignant hyperthermia, a genetic disorder triggered by certain anesthetics, cause uncontrolled heat production and demand urgent medical attention.