Fever in adults: When to worry
Infection is a common cause of fever, but other conditions can also bring the heat.
- Reviewed by Howard E. LeWine, MD, Chief Medical Editor, Harvard Health Publishing; Editorial Advisory Board Member, Harvard Health Publishing
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
Howard E. LeWine, MD, Chief Medical Editor, Harvard Health Publishing; Editorial Advisory Board Member, Harvard Health Publishing
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