A fever describes a condition where a patient's body temperature rises above 101 F (39 C), well above the normal range of body temperatures (96-100 F, 36-38 C). At this temperature, the patient clearly feels discomfort.
A fever is generally a sign that the patient is fighting off an infection, although it may merely mean the patient is unable to regulate their own body temperature or has overheated from exertion or exposure to extremely high temperatures. However, with the last two conditions, the body temperature quickly returns to normal along with the temperature of the environment. With an infection, the higher temperature usually retards the infection as most infective agents in humans require a suitable constant temperature near normal human body temperature in order to thrive. This explains why humans rarely get infections diseases that affect other animals, which often have radically different body temperatures. For example, 39 C would be well within the normal range for a house cat.
Usually, a fever will run its course once the body's antibodies and other parts of the immune system fight off the infection. However, during this period, a patient must be kept well hydrated with fluids as a patient will sweat profusely to attempt to control their body temperature. In addition, temperatures in excess of 105 F (41 C) are generally fatal to a patient, requiring that the patient be cooled as soon as possible.