Health Encyclopedia


What is a fever?

A fever is a body temperature that is higher than normal. It usually means there is an abnormal process occurring in the body. Exercise, hot weather, and common childhood immunizations can also 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 medication. Becoming overheated at play or in the sun can also result in fever.

What are the symptoms of a fever?

Normal body temperature ranges from 97.5°F to 98.9°F (36.4°C to 37.2°C). It tends to be lower in the morning and higher in the evening. Most health care providers consider a fever to be 100.4°F (38°C) or higher. 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, and/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, including the following:
  • Digital thermometer (oral, rectal, or under the armpit)
  • Ear thermometer (not recommended in babies younger than 3 months of age)

Taking a temperature rectally is the most accurate method in children under 3 years of age. Taking the temperature under the armpit or in the mouth can be used for older children and adults. Talk with your health care provider about the best way to take your temperature.

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

How is a fever treated?

You can treat a fever with acetaminophen or ibuprofen in dosages advised by your health care provider. Switching between giving acetaminophen and ibuprofen can cause medication errors and may lead to side effects. Never give aspirin to a child or young adult who has a fever.

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

Call your health care provider for guidance anytime you are uncomfortable with the conditions of the fever, and remember to contact your health care provider any time a temperature spikes quickly or persists despite treatment.

When should I call my health care provider?

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

Call right away if any of the following occur with a fever:

  • Seizure
  • Feeling dull or sleepy
  • Irregular breathing
  • Stiff neck
  • Confusion
  • Purple spotted rash
  • Ear pain (a child tugging on his or her ear)
  • Sore throat that persists
  • Vomiting
  • Diarrhea
  • Painful, burning, or frequent urination

Key points about fevers

  • A fever is not an illness by itself, but, rather, a sign that something is not right within the body.
  • Illness, exercise, hot weather, and common childhood immunizations 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 dosages advised by your health care provider.
  • Call your health care provider if a baby under 3 months has a fever, or if a fever is accompanied by 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 health care provider:

  • 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 names of new medicines, treatments, or tests, and any new instructions your provider gives you.
  • 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.

Medical Reviewers:

  • Bowers, Nancy, RN, BSN, MPH
  • newMentor board-certified, academically affiliated clinician