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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. Normal body temperature is maintained by temperature control activities in the body that keep a balance between heat loss and heat production.

An abnormal increase in body temperature is caused by either hyperthermia (an unusual increase in body temperature above normal) or fever. Hyperthermia is caused by a breakdown in the body's temperature control activities. In fever, the temperature controls in the body are working correctly, but body temperature increases as the body responds to chemicals produced by microorganisms that cause infection or works to kill harmful microorganisms such as bacteria or viruses. There are three phases to fever. In the first phase, the body raises its temperature to a new level by causing the blood vessels in the skin to constrict and move blood from the skin surface to the interior of the body which helps to retain heat. The skin becomes cool, the muscles contract causing shivering or chills, and the body produces more heat. The body's efforts to retain and produce heat continue until a new higher temperature is reached. In the second phase, heat production and heat loss are equal, shivering stops, and the body maintains the new higher temperature. In the third phase, body temperature is lowered to normal as the body gets rid of the excess heat by causing the blood vessels in the skin to open and move blood from the interior of the body to the skin surface. Sweating occurs and helps to cool the body.

Fever is most likely to cause harmful effects in older persons or the very young. In older persons, the hypothalamus' temperature regulating centers do not work as well and the body temperature may rise above normal causing irregular heartbeat, lack of blood flow, changes in the ability to think clearly, or heart failure. Children between 6 months and 6 years old may have seizures due to a fever.

To read the rest of this article from the National Cancer Institute, please click here: http://www.cancer.gov/cancertopics/pdq/supportivecare/fever/Patient/page2