This document provides information on fever (pyrexia), including: 1. Thermoregulation and how the body maintains a normal temperature of 37°C through the hypothalamus despite environmental variations. 2. Features of a normal body temperature and what constitutes a fever. Fever enhances the immune system and inhibits some microbes. 3. Patterns of fever onset, main phases, and end. Fever's diagnostic features and the importance of taking a thorough medical history are also outlined.