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Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

What Is It?

A fever is an increase in body temperature above the normal range. However, body temperature varies between people, with different levels of activity and at different times of the day. Medical textbooks differ in their definition of the highest normal body temperature. Fever generally can be defined as an early morning temperature higher than 99 degrees Fahrenheit or a temperature higher than 100 degrees Fahrenheit at any time of the day.

A part of the brain called the hypothalamus acts as the body's thermostat. When all is well in the body, the hypothalamus is set at the normal body temperature. Fever develops when the hypothalamus is set to a higher-than-normal temperature. This resetting of the hypothalamus is usually caused by small molecules called pyrogens in the blood. Pyrogens can come from outside the body (external) or can be produced inside the body (internal). External pyrogens include toxins (poisons) produced by infectious viruses or bacteria. Internal pyrogens include abnormal chemicals that are produced by tumors and proteins that are released during the normal response of the immune system.

Causes of fever include:

  • Hundreds of types of viruses, bacteria and parasites that cause many illnesses, such as upper respiratory infections, pneumonia, diarrhea and urinary tract infections

  • Chronic (long-term) conditions associated with inflammation, such as rheumatoid arthritis

  • Severe trauma, including surgery

  • Reactions to medications or immunizations

  • Certain types of cancers

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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