Fever is a rise in body temperature caused by the body's response to illness.
Normal human body temperature changes during each 24-hour period according to a definite pattern. It is lowest in the morning before dawn and highest in the afternoon. Temperature control actions in the body keep the amount of heat that is made equal to the amount lost. This keeps body temperature normal.
An abnormal rise in body temperature is caused by either a condition called hyperthermia or fever. Hyperthermia is caused by a breakdown in the body's temperature control actions. In fever, the temperature controls in the body are working as they should, but body temperature rises as the body responds to illness.
There are three phases of fever:
- In the first phase, body temperature rises as blood vessels in the skin narrow. This prevents heat from leaving the body through the skin. The skin becomes cool, the muscles contract and cause shivering or chills, and the body makes more heat. The body continues to make and keep heat until a new, higher temperature is reached.
- In the second phase, a new, higher temperature has been reached. The amount of heat the body makes and loses is the same. Shivering stops, and the body stays at the new, higher temperature.
- In the third phase, body temperature falls to normal as blood vessels in the skin open and move blood from inside the body to the skin surface. This helps get rid of extra heat. Sweating occurs and helps to cool the body.
Certain problems are more likely in older people or the very young. In older people, the temperature control centers in the brain may not work the way they should and can lead to hyperthermia. This may cause irregular heartbeat, lack of blood flow to parts of the body, confusion, or heart failure. In children between 6 months and 6 years old, high fever may lead to seizures.