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
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
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