What Is Considered a Fever (Pyrexia)?

A fever, also called pyrexia, is characterized by a temporary rise in body temperature. A body temperature of 100–100.4 degrees is what healthcare providers consider a fever in adults. An oral temperature of 99.5 degrees or a rectal temperature of 100.4 degrees is a fever in infants and children.

A fever is a common COVID-19 symptom. Research has found that people with COVID-19 may have a 98.9–100.6-degree fever. The only way to know if COVID-19 is the cause of your fever is to receive a test.

What's a Fever?

A fever is a higher-than-normal body temperature. Your average body temperature varies but is usually about 98.6 degrees. Some evidence suggests that the average body temperature has dropped to 97.9 degrees over the last two centuries.

A fever may be different in adults, children, and infants:

A fever is not a disease but typically means your body is fighting an infection. Bacteria and viruses that cause infections thrive in a normal body temperature. A fever results from your body trying to get rid of the invader by making your body an inhospitable environment for it. A fever starts an immune response, which defends you against the invader.

Autoimmune diseases, early-stage cancer, heat illness, and some medicines (i.e., antibiotics) may also cause a fever. Sometimes, children develop a low-grade fever after getting a vaccine.

Is 99 Considered a Fever for COVID-19?

A low-grade fever may range between 99.1–100.4 degrees, so a 99-degree fever may be an early sign of COVID-19. A review published in 2021 noted that 19% of studies that identified fever as a COVID-19 symptom described an elevated body temperature ranging from 98.9–100.6 degrees.

The researchers noted that there's no clear definition of what a fever for COVID-19 is. The only way to know if COVID-19 is the cause of your fever is to receive a test. The CDC advises getting tested right away if you have COVID-19 symptoms, including a fever. Begin self-isolation right away if you test positive. 

Isolate for at least five days, with day zero being when your fever—or other COVID-19 symptoms—began. You are most contagious during that time, so wear a high-quality mask if you must be around others in your household. You may end self-isolation after five days if you do not have a fever for at least 24 hours without using a fever reducer.

What's a Low-Grade Fever?

There are different ranges and severities of fevers, Jill Grimes, MD, a family physician at UT Austin's Student Health Services, told Health. For example, a low-grade fever is a body temperature above normal but not above or just barely above the fever threshold. A low-grade fever typically ranges between 99.1 and 100.4 degrees in adults and children.

Still, that's "not a true fever," said Dr. Grimes. In contrast, a low-grade fever in infants, slightly elevated but not quite 100.4 degrees, may be a sign of infection.

How To Take a Temperature

A fever may be present if you are hot to the touch, feel feverish, have chills and flushed skin, or look glassy-eyed. A body temperature reading using a digital or electronic forehead thermometer is the best diagnostic tool to determine a fever.

To take your temperature orally:

1. Clean your thermometer with cool water and soap or rubbing alcohol.
2. Place the thermometer under your tongue, then close your mouth. Hold it in place using your lips. Do not bite down on the probe. 
3. Keep the thermometer in your mouth for three minutes or until it beeps. 
4. Clean your thermometer once you are finished. 

You can take your temperature axillary by placing the thermometer under your armpit for five minutes. Other methods include electronic ear thermometers and plastic strips that change colors if you have a fever. Those methods are typically less accurate than a digital thermometer.

Infants and Children

Experts advise taking an infant or young child's temperature rectally since they cannot hold a digital thermometer firmly in their mouths. Keep a separate digital thermometer for rectal temperatures and label it as such. You can also take a child's temperature axillary or orally.

To take a rectal temperature:

1. Clean your thermometer with cool water and soap or rubbing alcohol.
2. Apply petroleum jelly to the probe of the thermometer.
3. Lay your infant or child across your lap and on their stomach.
4. Place the thermometer about half an inch into your infant or child's rectum, stopping if there's resistance. 
5. Keep the thermometer still while it takes the temperature. Remove it once it beeps.
6. Clean your thermometer once you are finished. 

Do not bundle up an infant or child in blankets or clothing or bathe them before taking their temperature. You may raise their body temperature, resulting in an inaccurate reading. Do not leave an infant or child alone with a digital thermometer, which may be a choking hazard.

Fever Symptoms 

Fever symptoms typically depend on how high your body temperature is, Matthew Mintz, MD, an internist based in Maryland, told Health. Fevers can be cyclical, meaning they go up and down. For example, you may begin to sweat when a fever breaks or comes down.

Some of the most common signs and symptoms of a fever include:

• Chills
• Flushed skin that is warm to the touch
• Increased heart rate
• Muscle spasms
• Sweating (e.g., night sweats)

Infants and children may show other signs and symptoms of a fever, such as:

• A lack of appetite
• Earache (e.g., a child may pull at their ears)
• Fussiness, high-pitching crying, and whimpering
• Painful, swollen joints
• Pale skin
• Severe headache
• Skin rash
• Sore throat
• Swollen soft spot on an infant's head
• Upset stomach

At-Home Treatment

A slightly elevated body temperature is not typically a cause for concern. You may want to take steps to lower, rather than suppress, your fever.

Here are a few things you can do to help make yourself comfortable while the fever runs its course:

• Avoid bundling up in blankets or excess clothing.
• Do not force food if you or your child do not have an appetite, but eat if you are hungry. 
• Get plenty of rest.
• Keep your room comfortable. Use a fan to cool down the room if it's too hot.
• Stay hydrated with gelatin, ice pops, soup, and water.
• Take fever reducers like Advil (ibuprofen), aspirin, and Tylenol (acetaminophen). Follow the instructions on the label for using fever reducers. Do not give Advil to an infant younger than 6 months or aspirin to a child younger than 18.

Follow the instructions on the label to give your infant or child the correct amount of medicine. Contact a healthcare provider if your infant is younger than 3 months before you give them medicine. Use a device, such as a syringe, to measure how much liquid medicine to give your child. 

You may give your child Children's Tylenol, then give them a warm bath. A bath may cause shivers and raise your child's temperature without medicine first.

When To Contact a Healthcare Provider

Contact a healthcare provider if you have a 103–105 degrees or higher fever that does not go away with at-home treatments and persists for longer than 48–72 hours.

For infants and children, contact a healthcare provider if:

• Your infant is 3 months or younger with a rectal temperature of 100.4 degrees or higher
• Your infant is 3–12 months with a 102.2-degree or higher fever
• Your child has a 105-degree or higher fever that does not go away with at-home treatments
• Your child is 2 or younger with a fever that does not go away after 24–48
• Your child is older than 2 with a fever longer than 48–72 hours

Call a healthcare provider right away if you or your infant or child have:   

• A cough, earache, or sore throat (for infants and children)
• A fever that comes and goes for one week or longer
• A weak immune system (i.e., due to cancer treatment or spleen removal)
• An underlying health condition (i.e., a chronic lung problem, diabetes, and heart disease)
• New bruises or rash
• Pain while urinating
• Recently been vaccinated (for infants and children)
• Recently traveled abroad

When To Seek Immediate Medical Attention

Get medical attention right away if you or your infant or child has:

• A stiff neck
• An inability to wake up
• An inability to walk
• Blue lips, nails, or tongue
• Confusion
• Crying and cannot calm down (for infants and children)
• Seizures
• Trouble breathing
• Will not move their arms or legs (for infants and children)

A Quick Review

A fever is typically a sign of an illness or infection, such as COVID-19. Healthcare providers generally consider a body temperature of 100–100.4 degrees a fever in adults. Infants and children have a fever if they have an oral temperature of 99.5 degrees or a rectal temperature of 100.4 degrees.

Mild fevers typically resolve with at-home treatments, like getting plenty of rest and staying hydrated. Consult a healthcare provider if you or your child has a high fever of 103–105 degrees that does not resolve with treatment after a couple of days. Always contact a healthcare provider if your infant has a fever of 100.4 degrees. 

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.