What Is a Fever?

Medically Reviewed by Dan Brennan, MD on December 24, 2022
4 min read

You probably always heard that the average human body temperature is 98.6 F. But the reality is that a "normal" body temperature can fall within a wide range, from 97 F to 99 F. It’s usually lower in the morning and goes up during the day. It peaks in the late afternoon or evening, sometimes by as much as 1 or 2 degrees.

If you’re healthy, you don’t need to take your temperature regularly. But you should check it more often if you feel sick or if you think you might have come into contact with an illness such as COVID-19. Many who are infected by the coronavirus have a fever or a temperature that’s higher than usual. Most also have fatigue and a dry cough.

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 periods between 1860 and 2017. 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 don’t consider you to have a fever until your temperature is at or above 100.4 F. But you can be sick if it’s lower than that.

Older adults’ bodies don’t respond to illnesses the way younger people’s do. Serious infections tend to cause symptoms such as confusion or weight loss rather than fever in older people. Doctors consider a temperature that’s 2 degrees higher than usual to be a sign of infection.

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

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, so add that much to your reading. 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.

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

Call your doctor if your temperature is over 102 F and it doesn’t go down within an hour 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.