How To Recognize A Fever And When To Seek Help, For Every Age Group

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Medical News TodaySubscribeWhat is considered a fever? How to recognize one in every age group

Medically reviewed by Cynthia Taylor Chavoustie, MPAS, PA-C — Written by MaryAnn De Pietro, CRT — Updated on May 30, 2025

Temperature of a fever
Average temperature
Causes
How to measure
When to seek help
Summary

The temperature that constitutes a fever varies by age and the method of measuring it. Generally, doctors consider any body temperature over 100.4°F (38°C) a fever.

A fever is a body temperature that is higher than usual due to an infection or illness.

This article explores what constitutes a fever in various age groups, possible causes of fever, and at what point a person should seek medical attention.

What temperature is considered a fever?

A father touches his daughter's forehead while she reads a thermometer. They are sat at a table on a video call with a doctor. — Geber86/Getty Images

Generally, doctors consider a temperature of 100.4°F (38°C) or above to be a fever.

However, several factors can influence the threshold, including:

Individual differences

The typical body temperature for an adult is around 98.6°F (37°C), although this can vary slightly from person to person. Some people are naturally a little hotter or colder than this.

Any temperature that is persistently higher than usual for a given individual could be a sign of a fever, particularly if they have other symptoms that suggest an infection, such as:

chills
fatigue
body aches
headache
cough
stuffy or runny nose
burning sensation when urinating
diarrhea
skin discoloration

Age

The temperature that doctors consider a fever for adults and children is often the same, at 100.4°F (38°C).

However, some guidelines state that the threshold for children over 3 months old is slightly higher, at 101.3°F (38.5°C) or more.

Some guidelines also set the threshold a little lower for older adults.

Differences by method

There are several methods of taking a person’s body temperature with a thermometer, including:

rectally
orally
in the ear
in the armpit
on the forehead

Body temperature measurements can vary by as much as 0.9°F to 1.8°F (0.5°C to 1°C) depending on the anatomic site.

A 2021 review notes that what experts consider a fever in each case varies slightly. A medical textbook, Harrison’s Principles of Internal Medicine, defines it as follows:

Method	High temperature
Rectal	99.5°F to 100.9°F (37.5°C to 38.3°C)
Oral	more than 99°F (37.2°C) in the morning or 99.9°F (37.7°C) in late afternoon
Armpit	more than 99°F (37.2°C)

A rectal temperature is the most accurate measure of core body temperature, but other methods are less invasive and more convenient in many cases.

What is a typical body temperature?

The average body temperature is about 98.6°F (37°C). However, body temperatures can vary between individuals and may fluctuate by as much as 0.9°F (0.5°C) over the course of a day.

Possible causes of fever

The causes of a fever include:

Common infections

Various viral and bacterial infections can trigger a fever, such as:

flu
strep throat
urinary tract infections
gastroenteritis
COVID-19

Autoimmune conditions and inflammation

Conditions that affect the immune system in other ways can also result in fever.

For example, autoimmune conditions occur when the immune system mistakenly attacks healthy tissue. This attack can involve an elevated body temperature.

Examples of conditions that may cause this include:

rheumatoid arthritis
systemic lupus
rheumatic fever

Cancer

Some cancers can cause fever, including lymphoma.

Health experts believe that cancer cells produce chemical signals that increase body temperature, leading to a fever.

How to take a temperature

A thermometer can measure body temperature. However, do not use a glass or mercury thermometer. Mercury is toxic and could cause poisoning if the thermometer breaks.

Instead, doctors usually suggest using a digital thermometer. People can use these orally or in the armpit.

Oral temperature

To use a digital thermometer orally, follow the steps below:

Make sure the thermometer is clean.
Read the directions that came with the thermometer.
Turn the device on.
Place the tip of the thermometer under the tongue and keep the mouth closed.
Wait until the device beeps, which signals a reading.
Check the reading.

It is best to avoid eating or drinking anything before taking an oral temperature. The temperature of the food and drink may alter the results.

Armpit temperature

For babies, it can be easier to measure their temperature via the armpit. To do this:

Lie them down or hold them on the knees.
Put the thermometer in their armpit and gently hold their arm against their body.
Hold in place according to the thermometer’s instructions. Usually, this is around 15 seconds.
Read the display.

When to seek help

It is not always necessary to seek medical care for a fever, especially if it is low grade or only lasts a short time.

However, people should get immediate help if a person has:

a high grade fever of 102.4°F (39.1°C) or more
severe headache
pain or swelling in any area of the body
difficulty breathing
a stiff neck
a rash that does not disappear under a glass
confusion, delirium, or loss of consciousness
signs of severe dehydration, such as a lack of sweat or urination

Summary

The temperature that constitutes a fever varies slightly by age and where on the body someone takes the temperature.

Usually, though, an oral temperature over 100.4°F (38°C) counts as a fever.

In many cases, fever as a sole symptom is not dangerous. But it could be a sign of an infection, including COVID-19.

If a person is unsure of the cause of a fever, or if a fever occurs in a baby less than 3 months old, they should contact a doctor.

Urinary Tract Infection
Emergency Medicine
Immune System / Vaccines
Infectious Diseases / Bacteria / Viruses
Parenthood
COVID-19

How we reviewed this article:

SourcesMedical News Today has strict sourcing guidelines and relies on peer-reviewed studies, academic research institutions, and medical journals and associations. We only use quality, credible sources to ensure content accuracy and integrity. You can learn more about how we ensure our content is accurate and current by reading our editorial policy.

Balli S, et al. (2023). Physiology, fever.https://www.ncbi.nlm.nih.gov/books/NBK562334/
High temperature (fever) in adults. (2023).https://www.nhs.uk/conditions/fever-in-adults/
High temperature (fever) in children. (2024).https://www.nhs.uk/conditions/fever-in-children/
How to take your baby's temperature. (2023).https://www.nhs.uk/baby/health/how-to-take-your-babys-temperature/
How is body temperature regulated and what is fever? (2022).https://www.ncbi.nlm.nih.gov/books/NBK279457/
Mackowiak PA, et al. (2021). Defining fever.https://pmc.ncbi.nlm.nih.gov/articles/PMC8394829/
Steele GM, et al. (2018). Noninfectious causes of fever in adults.https://journals.lww.com/tnpj/Fulltext/2018/04000/Noninfectious_causes_of_fever_in_adults.6.aspx

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Medically reviewed by Cynthia Taylor Chavoustie, MPAS, PA-C — Written by MaryAnn De Pietro, CRT — Updated on May 30, 2025

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