Everything You Need to Know About Fever

Medically Reviewed By Jenneh Rishe, RN
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Fever is an increase in your body’s temperature to a range that is above typical levels. While normal body temperature naturally fluctuates throughout the day by a few degrees, a fever can indicate illness or infection. It is normal for body temperature to change throughout the day by a few degrees or less.

This is because various factors can increase your body temperature mildly and temporarily. Examples include menstrual cycles, eating, physical activity, medications, your surrounding temperature, and a strong emotional response.

However, as the body’s immune response causes body temperature to rise, a high temperature can indicate a more serious underlying condition, such as infection.

This article will explain the definition of a fever including temperature ranges, causes, symptoms, treatment, frequently asked questions, and more.

What is fever?

A child in a darkened room has their temperature taken with an oral thermometer. The thermometer lights up to show a reading of 100.8°F.
Jennifer Bogle/Stocksy United

A fever refers to a rise in body temperature above what clinicians consider to be the normal, everyday range of temperature.

Fever temperature ranges

The Centers for Disease Control and Prevention (CDC) defines a fever as a measured temperature of 100.4°F (37.8°C) or higher. This applies to both adults and children.

However, the reading from a thermometer can vary according to how you use it and where the reading is taken from. Types of temperature readings include oral, rectal, forehead, or underarm.

Different clinicians and medical institutions may also have different thresholds for what they consider to be a fever.

Below are the ranges of readings that clinicians may consider to constitute a fever, based on the bodily location of the temperature testing:

Testing locationTemperature in fahrenheit (°F)Temperature in celsius (°C)
rectum99.5–100.9°F or higher37.5–38.3°C or higher
mouth 99–99.9°F or higher37.2–37.7°C or higher
skin, for example, the armpit99°F or higher37.2°C or higher

A fever may be present with a lower temperature in some older adults.

The CDC also notes that in some cases, it is not always possible to take an individual’s temperature.

In these cases, symptoms including skin that feels warm to the touch or a history of feeling feverish can indicate fever.

Clinicians refer to fever with a body temperature of above 106.7°F (41.5ºC) as hyperpyrexia, which is a life threatening emergency.

Read on here for more information about normal body temperature ranges and more.

What other symptoms might occur with fever?

Other symptoms may accompany a fever, depending on the underlying illness, disorder, or condition.

Fever is usually a sign of infection, which can lead to a variety of symptoms.

Fever may occur with other symptoms including:

Call 911 for serious symptoms with fever

In some cases, fever may occur with other symptoms that might indicate a serious or life threatening condition that requires immediate evaluation in an emergency setting.

Seek immediate medical care or call 911 for any of the following symptoms alongside a fever:

  • a change in level of consciousness or alertness, such as difficulty waking
  • a change in mental state or sudden change in behavior, such as confusion, lethargy, hallucinations, delirium, and delusions
  • pain in the abdomen, nausea, vomiting, or diarrhea
  • blood in urine, stool, or mucus
  • breathing problems, such as difficulty breathing, shortness of breath, wheezing, or choking
  • rapid heart rate
  • cyanosis, which is discoloration of the skin, particularly around the lips, fingernails, or toes
  • stiff neck
  • a fever of 104°F (40°C) or higher
  • muscle spasms
  • seizures, also known as febrile seizures when experienced with fever
  • difficulty moving or walking, such as children refusing to move a limb

When should you contact a doctor for fever?

A fever is not necessarily always a reason to contact your doctor. Most causes of mild fever are not serious, and the problem can sometimes resolve on its own.

However, there are cases that require medical advice and attention.

Although it is very rare for a fever to reach this temperature, fevers of 108°F (42°C) or more can cause brain damage. This primarily happens if the air temperature is very high. For example, this can occur in a child who is left in a hot car.

Fever in children

Contact the child’s doctor immediately under circumstances including:

  • a temperature of 100.4°F (38°C) or more in a child 3 months old or younger
  • a fever of 105°F (40°C) or above in a child of any age
  • a fever that persists for more than 4 days
  • a fever in an infant 12 months old or younger that lasts more than 2 days
  • a fever accompanied by additional symptoms of illness, including ear pain, cough, a new rash or bruises, or pain with urination
  • a fever in a child who has a weakened immune system, has a serious or long-term medical illness such as diabetes, or is taking medications that suppress the immune system
  • a fever in a child who has recently traveled internationally
  • symptoms that are worsening despite home treatment

Call 911 or go to your nearest emergency room for fever when the child presents serious symptoms alongside the fever, such as seizures, unexplained rash or bruising, stiff neck, or signs of dehydration.

Fever may not be serious if the child is still eating and drinking well, is alert and smiling, is still interested in play or entertainment, and appears generally well after their temperature has decreased.

Learn more about high fevers, including temperature ranges, causes, treatment, and more.

Fever in adults

Adults should contact their doctor for a fever in the following circumstances:

  • a fever of 104°F (40°C) or higher
  • a fever that lasts for 3 days or more
  • other symptoms that occur alongside fever
  • a fever in an adult who has a weakened immune system, has a serious or long-term medical illness such as diabetes, or is taking medications that suppress the immune system
  • a fever in someone who has recently traveled internationally
  • worsening symptoms despite home treatment

Call 911 or go to your nearest emergency room for fever when experiencing serious symptoms alongside fever.

What causes fever?

Many conditions can lead to a fever, including:

Infections have the potential to cause a fever. These include a common cold or influenza, sepsis, pneumonia, meningitis, and tuberculosis.

As there are so many possible causes of fever, it is important to contact your doctor to address your symptoms and make sure you receive appropriate care to avoid complications.

In some cases, fever may be a symptom of a serious or life threatening condition that requires immediate evaluation in an emergency setting.

How do doctors diagnose the cause of fever?

The initial evaluation of a fever will start with a physical exam and medical history.

The exam will focus on potential sources of the fever.

The doctor will likely pay special attention to the ears, nose, throat, skin, and abdomen. Your doctor may also listen to your heart and lungs.

Questions for diagnosing the cause of fever

To diagnose the underlying cause of fever, your doctor may ask you several questions related to your symptoms. Questions for diagnosing the cause of fever include:

  • When did the fever start?
  • How long have you had a fever?
  • Is the fever constant or intermittent?
  • Is the fever reduced by common over-the-counter medications, such as acetaminophen (Tylenol)?
  • Do you have any other symptoms, such as a cough or sore throat?
  • Have you recently traveled outside the country or been around anyone who is sick?
  • Have you recently had surgery or an immunization?
  • What other medical conditions do you have?
  • What medications do you take?

Depending on your answers and the results of the physical exam, your doctor may order further testing.

This could include nose or throat swabs, blood tests, urinalysis, or chest X-ray.

What are the treatments for fever?

Treatment will depend on the cause of the fever and your symptoms. This is because fever is a symptom of underlying illness, not an illness itself.

Low grade fever

Low grade fevers can actually be helpful to the body for dealing with some infections and can improve in a few days. Because of this, mild fevers may not require treatment to make the fever go away.

Treatment in these cases can instead focus on improving comfort and alleviating symptoms for the person or child with a fever.

High fever

However, high fevers may need medical care and some underlying causes of fever may need treatment.

Treatment of fevers can include the use of antipyretics, which are medications that can reduce fever. These medications can include acetaminophen (Tylenol) and ibuprofen (Advil, Motrin). Follow dosing instructions on the bottle or contact your doctor or pharmacist for advice.

Do not give aspirin to children younger than 16 years old. Do not give ibuprofen or acetaminophen to children younger than 3 months.

Read on here for more information about low grade fevers.

Home remedies for fever

There are several home remedies for mild fever to help keep you comfortable. These methods can include:

  • removing excess layers such as clothing or blankets
  • making sure the room’s temperature is not too hot
  • staying hydrated by drinking plenty of fluids, preferably water, and avoiding alcohol
  • taking acetaminophen or ibuprofen for any discomfort
  • avoiding contact with other people until you no longer have a high temperature

Do not attempt to make yourself feel cold, such as with a cold shower, if you feel hot with a fever.

Other frequently asked questions

Below are some more commonly asked questions about fevers. The answers have been reviewed by Jenneh Rishe, R.N.

How many degrees do you add when you take a temperature under your arm?

An axillary temperature, a temperature reading taken from the armpit, can give slightly lower readings than other body sites, such as from the mouth or rectum.

While clinicians generally consider a rectal, ear, or forehead temperature reading of 100.4°F (38°C) or more to be a fever, an axillary temperature reading of 99°F (37°C) is a fever. This is slightly less than 1ºF of difference.

What is a high temperature for COVID-19?

COVID-19 can lead to a fever as your body deals with the underlying infection.

A high temperature, no matter the cause, usually includes temperatures of 100.4°F (38°C) or higher in both children and adults.

Is 99.5°F a fever in adults?

The definition of a fever can vary by medical institution. Many healthcare bodies, such as the CDC, define fever as a body temperature of 100.4°F (38°C) or higher.

However, others define body temperature above 99.5°F (37.5°C) as a fever.

Summary

Fever refers to a body temperature of 100.4°F (38°C) or more.

Fever is a symptom of the body’s reaction to an underlying cause. These causes can include infection, inflammatory diseases, certain medications, blood clots, and cancer, among other factors.

Mild fevers without additional symptoms can be uncomfortable, but they generally resolve on their own without clinical treatment. At-home treatments can include staying hydrated with water and taking acetaminophen or ibuprofen if needed.

Severe symptoms of fever require emergency care. Seek emergency medical treatment or call 911 for a fever of 104°F (40°C) or more that occurs alongside symptoms including seizures, stiff neck, breathing problems, or a change in consciousness or alertness.

If you are unsure about care or the seriousness of the condition, contact your doctor for advice.

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Medical Reviewer: Jenneh Rishe, RN
Last Review Date: 2022 May 30
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THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis or treatment. Never ignore professional medical advice in seeking treatment because of something you have read on the site. If you think you may have a medical emergency, immediately call your doctor or dial 911.