Fever: Symptoms, Temperatures, and What to Do

Medically Reviewed by Jabeen Begum, MD on June 20, 2024
Written by Frances Gatta, Hallie Levine, Kristin Mitchell
10 min read

A fever is when your body temperature is higher than usual, often due to an infection or illness. Doctors generally consider a fever to be an oral temperature of 100 F (37.8 C) or higher. 

When you have an infection or illness, your body releases substances that tell its temperature regulator (located in the brain) to adjust to a higher setting, raising your body temperature. 

 

The body's normal temperature is usually defined as around 98.6 F. But the reality is that a "normal" body temperature can fall within a wide range, from 97 F to 99 F. This number changes due to your hormones, activity levels, sleeping and waking up, and other things that happen in your body in 24 hours.

Your temperature is usually lower in the morning and goes up during the day. It peaks late afternoon or evening, sometimes by as much as 1 or 2 degrees.

You don’t need to take your temperature regularly if you're healthy. But you should check it more often if you feel sick or think you might have come into contact with an illness such as COVID-19. 

The 98.6 F standard dates to the mid-1800s. German doctor Carl Wunderlich measured the armpit temperatures of about 25,000 people and came up with an average of 98.6 F.

Newer research suggests that the number has since gone down. In a recent review, scientists looked at temperature records from three time periods spanning about 160 years. The average oral temperature slowly fell by about 1 degree to 97.5 F. A person’s age, gender, or weight didn’t make a difference, nor did the time of day.

Doctors have several ideas about why body temperatures are falling. They include:

  • Lower metabolic rates. Your body uses energy so all your systems can work the way they should. This creates heat. But people may have lower metabolic rates now because we weigh more than people did centuries ago. The less heat your body makes, the lower your temperature.
  • Lower rates of infection. In the 19th century, infections such as tuberculosis, syphilis, and long-term gum disease were more common. As a result, many people had higher body temperatures.
  • Better thermometers. We may have more accurate thermometers than people did a century ago.

Despite the new research, doctors generally don’t consider you to have a fever until your temperature is at or above 100 F. But you can be sick if it’s lower than that.

Over-the-counter medicines like acetaminophen, aspirin, ibuprofen, and naproxen, which are fever reducers, might hide a high temperature if you take them for another symptom like pain.

 

 

Doctors use different names to describe fevers based on how they happen.

Intermittent fever

With intermittent fever, the temperature goes up but then returns to normal daily.

Remittent fever

Though your temperature falls each day with this type, it doesn’t return to normal.

Continuous or sustained fever

Your elevated body temperature stays high and only changes by a few degrees in 24 hours.

Hectic fever

Hectic fever causes big swings in temperature and usually comes with chills and sweats.

Relapsing fever

Relapsing fever causes episodes of serious rise in your body temperature, with periods of normal temperature in between that can last for days or weeks.

What temperature is a fever?

Fever can be grouped into low-grade, mid-grade, high-grade, and hyperthermia.

Low-grade. You have a low-grade fever when your body temperature ranges from 99.1 to 100.4 F. Having a low-grade fever might mean that your immune system has, to an extent, been activated.

Moderate-grade. Your doctor might describe your fever as moderate-grade if it falls between 100.6 to 102.2 F. Adults with chronic health conditions, like heart or lung problems or dementia, may notice their symptoms worsen when they have a moderate-grade fever.

High-grade. A body temperature that falls between 102.4 and 105.8 F is a high-grade fever. Sometimes, you can have a high fever from only a cold or other viral infection.

Hyperthermia. This is a fever over 105.8 F. It happens when your body temperature jumps so high because your body’s temperature regulator can’t handle how hot your environment is. Hyperthermia may show up as heat syncope or dizziness, heat cramps, heat exhaustion, and heat stroke. It can also happen when you have a life-threatening complication from an infection called sepsis or as a side effect of a medication. 

When you have other symptoms with fever, it’s usually because of the condition causing it. Other symptoms you might have when you have a fever include:

  • Feeling cold and shivering
  • Headache
  • Body pain
  • Sweating
  • Increased heart rate
  • Tiredness
  • Having flushed or hot skin

If you're an older adult, serious infections tend to cause symptoms such as confusion or weight loss rather than fever. Doctors consider a temperature that’s 2 degrees higher than usual to be a sign of infection.

Children may show other symptoms like

  • Being unable to eat or drink
  • Having pain in their ears
  • Feeling too thirsty
  • Not peeing as much as they used to 
  • Looking pale or flushed

Most cases of fever happen when you have an infection. Your body is trying to fight off the virus or bacteria that caused the infection. 

Most bacteria and viruses survive when your temperature is normal, but surviving becomes tougher for them with a fever. Your immune system also becomes activated to fight off the germ when you have a fever.

If you have a fever that lasts for 4 days or less, it's likely caused by an infection. If your fever sticks around for a while or keeps coming back, it's more likely to be caused by something else.

These causes include: 

  • Medicines, like anti-seizure and blood pressure medicines, or antibiotics
  • Certain types of cancer
  • Heat illnesses like heat stroke and heat exhaustion
  • Teething in children
  • Some vaccines given to children
  • Autoimmune or inflammatory diseases like inflammatory bowel disease (IBD)

A thermometer is the only way to know that you have a fever. Touch tests and skin pinching aren’t reliable. Rectal thermometers, which go into your rear end, are the most accurate, but they can be uncomfortable. Armpit, ear, and forehead thermometers aren’t as accurate. Most doctors think an oral thermometer -- which you hold under your tongue -- is best. Don’t use an old glass thermometer. These contain mercury, which is dangerous.

Before you use an oral thermometer, wash your hands with soap and warm water. Don’t eat or drink anything for at least 5 minutes before you take your temperature. Put the tip of the thermometer under your tongue. Keep your mouth closed. After about 30 or 40 seconds, the thermometer will beep. That means the final reading is ready. Oral thermometer temperatures are about 1/2 to 1 degree cooler than rectal ones. When you’re done, rinse the thermometer in cold water, clean it with alcohol, and rinse again.

If you have a child younger than 3, a rectal thermometer may be easier and more accurate. Put a small amount of lubricant like petroleum jelly on its tip. Have your child lie on their belly, and insert the thermometer into their bottom until the tip is completely inside. Don’t force it. When you hear the beep, after about 30 seconds, remove it. Check it and then clean it again.

A fever is a sign of your body fighting off an infection. As the actual fever isn't harmful, there's conflicting opinion about whether a fever needs to be treated. 

However, if your fever is higher than about 105.8 F or you have health conditions like lung disease, heart disease, or dementia, you need medication to break the fever. 

Medications for fever

Medication won't get rid of the infection quicker, but it can help with the discomfort that comes with a fever.

If your fever is higher than 101 degrees or is making you uncomfortable, you can try an over-the-counter medication.

These fever-reducing medicines include:

  • Acetaminophen
  • Nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen

If your child has a fever of 100.4 F or higher or they don't feel well, you can try children's doses of acetaminophen or ibuprofen. Follow the recommended dose on the medicine information leaflet. 

If the fever lasts longer than 3 days, or if your child is 3 months of age or younger with a fever of 100.4 or higher, get medical treatment immediately.

Don't give your child aspirin, as it can cause Reye syndrome, a serious condition that causes swelling in the liver and brain.

Home remedies for fever

You can cool down your body temperature using cooling blankets and dressing lightly, for example, if your temperature is high. 

Drinking plenty of fluids, resting, and having a lukewarm bath may also help to bring body temperature down.

Your baby has a fever if their temperature is 100.4 F or higher. In infants 3 months old or younger, seek medical care right away.

Usually, a fever is not a cause for concern. Only about 1 out of 100 fever cases in children are tied to a serious health condition, like pneumonia. Most cases are due to a harmless virus that their bodies can handle alone, and they’ll feel better within 2-3 days.

When your child has a fever, they may look tired and pale. Their forehead and neck can be hot. They may cry more and not want to eat or drink anything.

Take your child to a doctor immediately or call 911 if they have symptoms like:

  • Fever of over 100.4 F 
  • Fever that comes and goes
  • Fever that lasts for more than 3 days
  • Febrile seizure or convulsion
  • Stiff neck
  • Restlessness
  • Confusion
  • Vomiting
  • Diarrhea
  • Stomach pain
  • Skin rash
  • Worsening symptoms since last doctor visit
  • Refusal to eat or drink

If your temperature is between 100 and 102, drink plenty of fluids and rest. You can take a fever reducer if you like.

Calling a doctor for fever in adults

Call your doctor if your temperature is over 102 F and it doesn’t go down within 1-2 hours after you take a fever-reducing medication.

If you have a fever with a cough or shortness of breath and think you might have come into contact with someone who has COVID-19, call your doctor to talk about the next steps.

Always call your doctor if you have any kind of fever along with a severe headache, a stiff neck, throat swelling, or confusion. They may be signs of a serious condition, such as strep throat or meningitis.

Even if you don’t have these symptoms, your doctor may tell you to take your temperature at certain times, like first thing in the morning or at night. You can record the readings and report back.

Calling a doctor for fever in children

In children, call a doctor immediately if your child is:

  • Upset and can't be calmed down
  • 3 months of age or younger with a fever of 100.4 or higher (seek emergency care)
  • Younger than 2 years of age with a fever of 100.4 or higher lasting more than 1 day
  • 2 years old or older with a fever of 100.4 that lasts longer than 3 days
  • Any age and has repeat fevers of above 104 F

Fever over 105.8 F left untreated can cause complications like organ malfunction, organ failure, and death.  

See a doctor immediately if you have a high fever with symptoms like:

  • Difficulty breathing 
  • Thinking problems
  • Confusion
  • Severe chest or stomach pain or headache
  • Stiff neck
  • Pain when peeing
  • Convulsions or seizures
  • Rashes or bruises
  • Loss of consciousness
  • Discharge when peeing
  • Swelling

Moderate fevers can worsen symptoms in people with lung or heart diseases, as it raises breathing and heart rate. It can also worsen symptoms in people with dementia. 

Fever can also cause febrile seizures in children, typically between 6 months and 5 years of age. But they most often happen between 12 and 18 months of age. These seizures are usually harmless and stop within a minute or two. Some may happen for only a few seconds. In some cases, they can last for more than 15 minutes.

Fever is usually a sign that your body is fighting an illness. But it may also happen when you have chronic conditions like autoimmune diseases and cancer or as a medication side effect. The fever often goes away on its own, but you can break it by taking over-the-counter medicines like acetaminophen or ibuprofen. See a doctor if you have a high fever and a fever that doesn’t go away within 5 days. If your child has a fever, emergency care may be needed, depending on their age. Also, see a doctor immediately if you have other symptoms like breathing problems, body pains, headache, and stiff neck.

Can allergies cause a fever? No allergies don’t cause fever. If you’re having allergy-like symptoms like a runny or stuffy nose with a fever, you might have a sinus infection. 

What’s a fever dream? A fever dream is a strange dream that you may have when you have a fever. It often involves negative emotions and fewer characters and interactions.

What’s considered a fever in children? In children, a temperature at or above 100.4 F taken rectally, 99.5 F taken orally, or 99 F taken under the arm is considered a fever.

What does a fever feel like? When you have a fever, you may feel hot inside and out. You may also feel chilly and cold and have aches all over.

What does fever mean? A fever means that your body temperature is higher than usual. 

What is a low-grade fever? A low-grade fever is a high body temperature ranging from 99.1 to 100.4 F.