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Quick Facts

Fever in Adults


The Manual's Editorial Staff

Last full review/revision Feb 2020| Content last modified Feb 2020
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What is a fever?

Fever is a body temperature higher than 100° F (37.8° C). Fever isn't just feeling hot or sweaty. To qualify as a fever, your temperature must be high, as measured by a thermometer.

Normal body temperature isn't the same in everybody, but it's usually around 98.6° F (37° C). Normal body temperature can be up to 1 degree above or below this in some people.

  • Most fevers in healthy people are caused by an infection from a virus and usually go away on their own in a few days

  • When you have a fever, your symptoms are mostly from what’s causing the fever, not the fever itself

  • A fever itself can’t hurt you unless it goes higher than about 106° F (41.1° C)

  • Doctors try to find out what’s causing the fever with a physical exam and sometimes a few tests

  • Medicines such as ibuprofen or acetaminophen can help lower your fever if it's making you uncomfortable

What causes fever in adults?

Many disorders can cause fever.

The most common causes are:

Less often, fever may be caused by:

With cancer and inflammatory disorders, the fever usually lasts a long time. The fever may be there all the time or come and go.

In older people, the cause of a fever is usually a bacterial infection, often in the lungs, urinary tract, skin, or other soft tissues. However, infections don't always cause fever in older people.

Fever of unknown origin

A fever of unknown origin (FUO) is a fever that:

  • Is at least 101° F (38.3° C)

  • Lasts for several weeks

  • Has no obvious cause that can be found by doctors

Doctors usually do many tests, including blood and urine tests and imaging tests, to look for an unusual infection or a cancer. If these test results are negative, doctors may need to test a sample of tissue (biopsy) from your liver, bone marrow, or other area that may have an infection.

What are the symptoms of fever in adults?

Fever is a symptom of an illness. By itself a fever usually doesn't cause many symptoms except:

  • When the fever starts, you may shiver all over and have chattering teeth like you're out in the cold (called "chills")

  • If you have a high fever, your skin will feel hot to the touch

  • When the fever goes away ("breaks") you may sweat a lot

You may also have symptoms of whatever caused your fever. For example, if your fever was caused by a chest infection, you may have a cough.

When should I see a doctor about a fever?

See a doctor right away if you have a fever and any of these warning signs:

  • A change in thinking ability, such as confusion

  • A headache, stiff neck, or both

  • A rash

  • Feeling faint or light-headed

  • Fast heart rate

  • Shortness of breath or fast breathing

  • A temperature higher than 104° F (40° C) or less than 95° F (35° C)

  • Recent travel to an area where a serious infectious disease, such as malaria, is common

  • Recent use of drugs that weaken your immune system (immunosuppressants)

If you don’t have any of these warning signs but you have a fever for more than 1 or 2 days, call your doctor. Depending on your age, symptoms, and health history, your doctor may have you come in to be checked and may even do tests to find out what's causing your fever. They may order:

  • Blood tests

  • Urine tests

  • Chest x-ray

  • Other tests, depending on your symptoms

If your fever lasts more than 3 or 4 days with or without other symptoms, visit your doctor.

How do doctors treat fever in adults?

Depending on how you feel and what your temperature is, your doctor may not need to treat your fever—the fever means your body is fighting the infection. Your doctor will give you treatment for whatever is causing the fever.

If the fever itself is making you uncomfortable, your doctor may give you medicines, such as acetaminophen or ibuprofen, that will lower your fever. Drinking more fluids and wearing cool clothes may also make you feel better.

If your fever is higher than 105.8° F (41.0° C), you may need to be admitted to the hospital. In the hospital you'll be given fluids by vein (IV) and cooling blankets.

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