Fever

What Is It?

A fever is an increase in body temperature above the normal range. However, body temperature varies between people, and also with different levels of activity, and at different times of the day. Modern medical textbooks differ in their definition of the highest normal body temperature. Fever generally can be defined as an early morning temperature greater than 99° Fahrenheit, or a temperature greater than 100° 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 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 conditions associated with inflammation, such as rheumatoid arthritis

• Severe trauma, including surgery

• Reactions to medications or immunizations

• Certain types of cancers

Symptoms

Common symptoms associated with fevers include sweating, shivering, headaches, muscle aches, poor appetite, rash, restlessness and general body weakness. High fevers can lead to symptoms of mental dysfunction, such as confusion, excessive sleepiness, irritability and convulsions (seizures).

Convulsions triggered by fever (febrile seizures) are common among children younger than 5 years old. These seizures usually occur at the beginning of an illness when the temperature rises rapidly. In infants and toddlers, febrile seizures typically produce muscle rigidity and generalized convulsions that last less than 15 minutes. The febrile seizure usually is followed by a long period of sleep.

Specific symptoms associated with a fever often can provide clues to identify the fever's cause. For example, a fever accompanied by vomiting and diarrhea may signal gastroenteritis, while a fever associated with coughing, shortness of breath and the presence of grayish-yellow phlegm may indicate pneumonia.