Fever in adults: When to worry

Infection is a common cause of fever, but other conditions can also bring the heat.

Serious sad aged woman holding a thermometer.

Fever is one of the body's most effective ways of fighting infection. It is part of your body's defense against infection-causing germs.

You get a fever most often when your body is trying to kill infectious invaders such as viruses or bacteria. A higher body temperature helps the immune system respond more vigorously to the attack, and makes it harder for these microbes to survive.

What is considered a fever in adults?

The average body temperature is 98.6° F (37°C). But "normal" body temperature varies from person to person. It also changes during the day, rising a bit after you eat or exercise. Body temperature is often higher in the afternoon than it is when you wake up in the morning.

Fever means a body temperature of 100.4° F (38°C) or higher.

High fever in adults

While any temperature above your normal temperature range is considered a fever, there are different levels of fever severity:

  • Low-grade: 99.1 to 100.4 F (37.3 to 38.0 C)
  • Moderate-grade: 100.6 to 102.2 F (38.1 to 39.0 C)
  • High-grade: 102.4 to 105.8 F (39.1 to 41 C).

When to worry about fever

If you have a fever over 104°F (40°C), you should call your doctor.

Seek medical help right away if you have a fever along with any of these symptoms:

  • seizure
  • loss of consciousness
  • confusion
  • stiff neck
  • trouble breathing
  • severe pain anywhere in the body
  • swelling or inflammation of any part of the body
  • vaginal discharge that is discolored or smells bad
  • pain when urinating or urine that smells bad.

Causes of fever

An infection, such as the flu, is the most common cause of fever.

Other conditions can also cause a fever. These include diseases that produce inflammation, such as rheumatoid arthritis; reactions to drugs or vaccines; and even certain types of cancers.

Accompanying symptoms

The following symptoms may also accompany fever:

  • sweats
  • chills
  • headache
  • achy muscles
  • lack of desire to eat
  • rash
  • restlessness
  • weakness.

A very high fever can cause confusion, extreme sleepiness, irritability, and seizures.

Diagnosing the cause of a fever

To help determine why you have a fever, your doctor will ask you about:

  • other symptoms such as coughing, abdominal pain, vomiting, diarrhea, or pain when urinating
  • recent surgeries or injuries
  • recent vaccinations
  • new drugs you may be taking
  • recent travel, particularly travel abroad.

How to treat a fever at home

By itself, fever is usually harmless, though a high fever can be miserable. These steps may help you feel better:

  • Drink plenty of fluids to help cool your body and prevent dehydration.
  • Eat light foods that are easy to digest.
  • Get plenty of rest.
  • Take ibuprofen (Advil, Motrin, or others), naproxen (Aleve, Naprosyn, or others), or acetaminophen (Tylenol, others) to help relieve head and body aches and lower your temperature.
  • Take a slightly warm (not cool) bath, or apply damp washcloths to the forehead and wrists.

Image: yacobchuck/Getty Images

About the Reviewer

photo of Howard E. LeWine, MD

Howard E. LeWine, MD, Chief Medical Editor, Harvard Health Publishing

Dr. Howard LeWine is a practicing internist at Brigham and Women’s Hospital in Boston, Chief Medical Editor at Harvard Health Publishing, and editor in chief of Harvard Men’s Health Watch. See Full Bio
View all posts by Howard E. LeWine, MD


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No content on this site, regardless of date, should ever be used as a substitute for direct medical advice from your doctor or other qualified clinician.

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