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What is a fever?

A fever is a body temperature that is higher than normal. It often means there is an abnormal process occurring in the body. Exercise, hot weather, inflammation, infection, vaccines, and many other things can make body temperature rise.

What causes a fever?

A fever is not an illness by itself. Rather, it is a symptom that something is not right within the body. A fever does not tell you what is causing it, or even that a disease is present. It may be a bacterial or viral infection. Or it could be a reaction from an allergy to food or medicine. Becoming overheated at play or in the sun can also result in fever. There are many other causes of fever as well.

What are the symptoms of a fever?

Normal body temperature ranges from 97.5°F to 99.5°F (36.4°C to 37.4°C). It tends to be lower in the morning and higher in the evening. Most healthcare providers consider a fever to be 100.4°F (38°C) or higher. A person with a temperature of 99.6°F to 100.3°F has a low-grade fever. High fevers may bring on seizures or confusion in children. It's not how high the temperature is but how fast the temperature goes up that causes a seizure.

A fever has other symptoms besides a higher-than-normal temperature. These are especially important when caring for babies, young children, and disabled people. These groups may not be able to express how they feel. Signs that mean fever include:

  • Flushed face

  • Hot, dry skin

  • Low output of urine, or dark urine

  • Not interested in eating

  • Constipation or diarrhea

  • Vomiting

  • Headache

  • Aching all over

  • Nausea

How is a fever diagnosed?

The best way to diagnose a fever is to take a temperature with a thermometer. There are several types of thermometers, such as:

  • Digital thermometer (oral, rectal, or under the armpit)

  • Tympanic (ear) thermometer (not recommended in babies younger than 6 months of age)

  • Temporal artery (temperature taken across the forehead area)

Taking a temperature rectally is the most accurate method in children younger than 3 years of age. In older children and adults, take the temperature under the armpit or in the mouth. Talk with your healthcare provider about the best way to take your temperature.

Mercury thermometers

Most thermometers today are digital. But there are some glass thermometers containing mercury still in use. Mercury is a toxic substance. It is dangerous to humans and the environment. Because glass thermometers can break, they should be disposed of in accordance with local, state, and federal laws. For information on how to safely dispose of a mercury thermometer, contact your local health department, waste disposal authority, or fire department.

If a mercury thermometer breaks, have everyone leave the area, including pets. Open all windows and doors to the outside. But shut all doors to the rest of the house. Make certain pregnant women leave the area. Call your local poison control center at 800-222-1222. They will tell you how to clean up the mercury. Don't let children help clean up the spill. Don't use a vacuum cleaner or broom to clean up the droplets. Don't pour mercury down a drain.

How is a fever treated?

You can treat a fever with acetaminophen or ibuprofen in doses advised by your healthcare provider. Switching between giving acetaminophen and ibuprofen can cause medicine errors and may lead to side effects. But it sometimes works better than using only one or the other. Never give aspirin to a child or young adult who has a fever. Aspirin can cause an upset stomach and intestinal bleeding. It may also cause Reye syndrome. This is a rare but very serious illness that can affect all organs of the body. But it most often injures the brain and the liver.

A lukewarm bath may reduce the fever. Alcohol rubdowns are no longer recommended.

Call your healthcare provider for guidance anytime you are concerned about a fever. Also call your healthcare provider if a temperature spikes quickly or lasts despite treatment.

When should I call my healthcare provider?

Call your healthcare provider right away for a fever in a baby younger than 3 months old.

Call or get medical care right away if any of these occur with a fever:

  • Seizure

  • Feeling dull or sleepy

  • Irregular breathing

  • Stiff neck

  • Confusion

  • Purple spotted rash

  • Ear pain (a child tugging on their ear)

  • Sore throat that doesn't go away

  • Vomiting

  • Diarrhea

  • Painful, burning, or frequent urination

Key points about fevers

  • A fever is not an illness by itself. Rather, it's a sign that something is not right within the body.

  • Illness, infection, exercise, hot weather, inflammation, vaccines, and many other things can make body temperature rise.

  • In addition to an elevated temperature, look for other signs, such as: flushed face, hot skin, low urine output, loss of appetite, headache, or other symptoms of an infection or illness.

  • Once you have determined that the person has a fever, you may treat it by giving acetaminophen or ibuprofen in doses advised by your healthcare provider. Never give aspirin to a child or teen who has a fever.

  • Call your healthcare provider if a baby younger than 3 months has a fever. Or get medical care right away if a fever occurs with a seizure, lethargy, irregular breathing, stiff neck, confusion, or other signs of a serious illness.

Next steps

Tips to help you get the most from a visit to your healthcare provider:

  • Know the reason for your visit and what you want to happen.

  • Before your visit, write down questions you want answered.

  • Bring someone with you to help you ask questions and remember what your provider tells you.

  • At the visit, write down the name of a new diagnosis and any new medicines, treatments, or tests. Also write down any new instructions your provider gives you.

  • Know why a new medicine or treatment is prescribed, and how it will help you. Also know what the side effects are.

  • Ask if your condition can be treated in other ways.

  • Know why a test or procedure is recommended and what the results could mean.

  • Know what to expect if you do not take the medicine or have the test or procedure.

  • If you have a follow-up appointment, write down the date, time, and purpose for that visit.

  • Know how you can contact your provider if you have questions.

Online Medical Reviewer: Barry Zingman MD
Online Medical Reviewer: L Renee Watson MSN RN
Online Medical Reviewer: Rita Sather RN
Date Last Reviewed: 2/1/2023
© 2000-2024 The StayWell Company, LLC. All rights reserved. This information is not intended as a substitute for professional medical care. Always follow your healthcare professional's instructions.