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Medically reviewed by C.H. Weaver M.D. Medical Editor 7/2021

Fever is the body’s natural response to infection. An infection-related fever may be particularly dangerous when your white blood cell count is low or is expected to be low, because this is when the body’s normal defenses against infection are low. Under these conditions, a fever needs to be carefully monitored and evaluated for a cause.

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What is a fever

Fever is an abnormally high body temperature. The average human body temperature is 98.6ºF (37ºC); however, each individual person is different and normal may range from 97ºF to 99ºF. Even under normal conditions, the human body temperature varies throughout the day — it is lower in the morning and higher in the late afternoon and evening.

Fever is the body’s natural response to infection. A part of the brain called the hypothalamus raises the body temperature to create an environment that is unfavorable for the bacteria or viruses that cause infectious disease. For example, the viruses that cause colds and other respiratory infections thrive at cool temperatures. One way the body works to eliminate a virus is by producing a fever.

How is a fever diagnosed

A fever is confirmed with 3 oral temperatures greater than 100.4ºF (38ºC) recorded in a 24-hour period, or with one oral temperature greater than 101.3ºF (38.5ºC). A severe fever is an oral temperature of 102ºF or a rectal temperature of 103ºF.

If the fever is infection-related, several tests may be carried out to determine the source of the infection. Samples of blood, urine, tissue from the throat, or drainage from a wound or catheter may be taken and grown in a laboratory culture. If an infection is present in one of these samples, the bacteria will replicate to a degree that can then be detected and identified by laboratory procedures.

What are the signs and symptoms of a fever

Depending on what is causing your fever, your signs and symptoms may include:

  • Sweating
  • Shivering
  • Headache
  • Muscle aches
  • Lack of appetite
  • Dehydration
  • General weakness
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Very high fevers, between 103ºF and 106ºF, may cause hallucinations, confusion, irritability and even convulsions.

Why is a fever important in a cancer patient

For a cancer patient, fever may be a sign of a dangerous infection. An infection may be particularly serious when your white blood cell count is low or is expected to be low. This is because white blood cells are the body’s normal defenses against infection, and when their numbers are low, the body’s ability to fight infection is reduced.

Additionally, many cancer patients develop a fever in which a definite source of infection cannot be found. Fever is a side effect of some biological therapies, and may occur in patients who are receiving chemotherapy and biological therapy as part of the “flu-like syndrome”. A fever associated with “flu-like syndrome” usually peaks at 104ºF (40ºC) and often spikes after a severe chill.

How is a fever treated

Medical treatment for a fever depends on the cause. If the fever is determined to be related to an infection, treatment will be prescribed for the infection. For a bacterial infection, such as pneumonia or tonsillitis, antibiotics are usually prescribed. For viral infections, including stomach flu (gastroenteritis), the best treatment is often rest and plenty of fluids, although anti-viral drugs may be used to treat some viruses. If an infection-related fever is tolerable, then doctors may not treat the infection but monitor it closely because the fever contributes to eliminating the bacteria or virus. If a cancer patient develops a low white blood count and fever, it is assumed that an infection is present and antibiotics are given until the fever is resolved and white blood cell count recovered.

If an infection-related fever is very high, over-the-counter medications may be prescribed to reduce the fever and associated discomfort. Non-steroidal anti-inflammatory drugs (NSAIDs), such as acetaminophen (Tylenol®) or ibuprofen (Advil®, Motrin®, etc.), are often used for this purpose. Adults may also use aspirin to reduce a fever. However, aspirin should not be administered to children as it may cause a rare, but potentially fatal disorder known as Reye’s syndrome.

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If you have a bleeding disorder, you should avoid NSAIDs, as well as aspirin. These drugs may prolong bleeding by interfering with the activity of blood platelets. Use of such drugs to treat fever should be discussed first with your healthcare professional.

NSAIDs or aspirin are also used if the source of the fever is related to flu-like syndrome associated with biologic therapy or certain types of chemotherapy and not infection.