Fever: Causes, Symptoms, and Treatment

Medically Reviewed by Zilpah Sheikh, MD on November 15, 2023
6 min read

A fever is when you have a brief rise in body temperature that’s higher than your normal body temperature. It’s also called a high temperature, hyperthermia, or pyrexia. It's usually a sign your immune system is working to fight some type of infection. 

Fever temperatures

Normal body temperatures are different for everyone, but they usually fall within the range of 97 to 99 degrees F. A temperature of 100.4 or higher is considered a fever, but there is no medical standard. The type of thermometer you use affects the temperature you get, as some are more accurate than others. 

Here's a breakdown of what's considered a fever by age and temperature: 

Babies age 0-2 years

  • 100.4 F (rectal, forehead, or ear temperature)
  • 99 F (under the arm temperature)

Children ages 2-5 years

  • 100.4 F (rectal, forehead, or ear temperature)
  • 99 F (under the arm temperature)

Children ages 5 and older

  • 100.4 F (ear temperature)
  • 100 F (oral temperature)
  • 99 F (under the arm temperature)


  • 100.4 F (oral temperature)

A low-grade fever is when you have a temperature that is slightly higher than normal, usually around 99.5 to 100.3 F. A low-grade fever signals that something is going on to activate your immune system.

A fever can make you uncomfortable, but it's generally not cause for concern. You can often manage a fever at home, but in some cases you should call your doctor. 

Fever in babies: When to worry
Check in with your doctor if your baby younger than 3 months:

  • Has a rectal temperature higher than 100.4 F. 
  • Has fever higher than 102 F for more than 1 day. 
  • Has other symptoms, such as a cough or diarrhea.
  • Call 911 or go to the ER if your baby has a seizure.

Check in with your doctor if your baby 3 to 6 months old: 

  • Has a rectal or oral temperature higher than 100.4 F and is also irritable or sleepy.
  • Has a rectal temperature higher than 102 F for more than 1 day. 
  • Call your doctor or go to the ER if your baby won't stop crying and can’t be consoled. 

Fever in children: When to worry
You should reach out to your pediatrician if your child 6 months to 2 years:

  • Has a fever higher than 102 F for more than 1 day. 
  • Has other symptoms, such as a cough or diarrhea.
  • If their fever comes with rash, real discomfort, irritability, low energy, headache, stiff neck, or repeated diarrhea or vomiting.
  • Their fever is higher than 104 F, which can cause a seizure.
  • Call 911 or go to the ER if your child has a seizure.
  • Their fever doesn’t go down after taking over-the-counter medications, such as ibuprofen.
  • They've recently had vaccines.

Fever in adults: When to worry
Call your doctor or visit urgent care or an emergency room if:

  • You have a temperature of 103 F or higher.
  • Your fever doesn’t go down after taking over-the-counter medications, such as ibuprofen.
  • You’ve been in contact with someone who has COVID-19.
  • If you have a fever that is 102 degrees or higher, especially if you're in your first trimester of pregnancy.

When to call the doctor

Call 911 if you or your child have any of these symptoms:

  • Unresponsive
  • Wheezing or difficulty breathing
  • Blue lips
  • Convulsions or seizures
  • Speaking in a confused way 
  • A fever with a stiff neck or headache
  • Temperature higher than 105 F
  • Fever with sudden onset of rash
  • Severe stomach pain or persistent vomiting
  • Skin rashes, blisters, or a red streak on an arm or leg
  • A severe sore throat, swelling of the throat, or earache
  • Pain with urination (peeing), back pain, or shaking chills
  • A serious cough, or coughing up blood

Contact your doctor if your fever lasts for more than 3 days or gets worse.

A fever is a sign of an illness or infection. When you have one, you may notice these symptoms:

  • Chills or shivering
  • Sweating
  • Body aches
  • Headache
  • Lethargy (feeling weak)
  • Being irritable
  • Loss of appetite
  • Dehydration
  • Red or flushed face

Babies and children might also have:

  • Earache (or pulling at their ears)
  • High-pitch cries
  • Loss of appetite
  • Extra thirst
  • Low urine output (peeing less than usual)

A fever can be a sign of several health conditions, which may or may not need medical treatment.

The most common causes are infections such as colds and stomach bugs (gastroenteritis). Other causes include:

  • Infections of the ear, lung, skin, throat, bladder, or kidney
  • Heat exhaustion
  • Viral infections like the flu or COVID-19
  • Sunburn
  • Conditions that cause inflammation, such as rheumatoid arthritis
  • Side effects of medications
  • Vaccines and immunizations
  • Blood clots
  • Autoimmune conditions such as lupus and inflammatory bowel disease (IBS)
  • Cancer
  • Hormone disorders such as hyperthyroidism
  • Illegal drugs such as amphetamines and cocaine
  • Teething in babies can cause a mild, low-grade fever (not over 101 degrees).

Although a fever is easy to measure with a thermometer, finding its cause can be hard. If you see your doctor, they'll do a physical exam and ask questions about your symptoms, other health conditions you have, medications you take, and if you've recently traveled to areas that have health risks. A malaria infection, for example, may cause a high fever that gets better and then comes back in cycles. Some areas of the U.S. are hot spots for infections such as Lyme disease and Rocky Mountain spotted fever.

Your doctor may ask if you have been around someone with COVID-19 or have any other symptoms of COVID-19.

Sometimes, you may have a "fever of unknown origin." That means whatever's causing your fever isn't clear. Instead, it could be more unusual like a connective tissue disorder (diseases that affect things like your tendons, ligaments, skin, or cartilage), cancer, or another health problem. Your doctor may need to run tests to figure out the cause.

The most common treatments for fever include over-the-counter drugs such as acetaminophen  and nonsteroidal anti-inflammatory (NSAID) drugs like ibuprofen and naproxen. Talk to your doctor about the proper dosage. Never give young children and teens aspirin. It's linked to a rare condition called Reye’s syndrome that could damage their liver and brain.

What is the best way to treat a fever at home?

  • Drink a lot of clear liquids such as water, broth, and juices. 
  • Take a lukewarm bath. Do not use cold water or alcohol.
  • Get plenty of rest. 
  • Keep yourself cool with lightweight clothing and bed coverings.
  • If you get the chills, use an extra blanket until they go away.

Fevers have a wide range of causes, and you might hear your doctor call your fever a certain name. That's because doctors classify five types of fevers based on their patterns.

Intermittent. With this type of fever, your temperature rises, but falls back to a normal range during the day. The difference can range from a small increase to about 2 degrees Fahrenheit. 

Remittent. This type of fever rises and falls, but your temperature always stays above normal. 

Continuous or sustained. Your temperature stays about the same throughout the day. 

Hectic. This causes big swings in temperature, about 2 degrees F or more, throughout the day. You may also have symptoms like chills and sweats. 

Relapsing. A relapsing fever spikes after a few days or even weeks of normal temperature.

Most fevers get better without any problems, but some can have rare complications, especially in young children.

If your child is aged 6 months to 5 years, a fever could put them at risk of a febrile seizure. They usually occur when your child's fever is higher than 100.4 F. 

Signs your child could be having a febrile seizure include: 

  • They lose consciousness.
  • Their arms and legs shake.
  • Stiff or twitching one side of their body.

Febrile seizures usually happen within the first 24 hours of your child's fever, and they might be the first sign your child is sick. Call 911 or your doctor immediately, even if the seizure lasts just a few seconds.