Version July 2025
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What is fever of unknown origin? —
Fever of unknown origin, or "FUO," is when a person has a fever with no clear cause.
A fever is a rise in body temperature that goes above a certain level. In general, this means 100.4°F (38.0°C) or higher. The most common cause of a fever is an infection. But other health problems can also cause a fever. These include rheumatologic conditions (such as arthritis or vasculitis), cancer, and certain medicines.
Doctors often define FUO as a fever that:
●Lasts for 3 weeks or longer
●Has no obvious cause based on testing
Often, the cause is never found, and the fever goes away on its own. In some cases, doctors do find a cause after testing.
Will I need tests? —
Yes. If you have a fever without a clear cause, your doctor or nurse will do an exam and ask you questions. This includes how long you have had a fever, whether you have other symptoms, past health problems, and what medicines you take. They will also ask about your recent activities, including things like travel or being around animals. All of this can help give clues about what might be causing your fever.
You will also get tests, such as:
●Blood tests
●Urine tests
●CT scan
Doctors diagnose FUO when these tests do not find a clear cause. In some cases, they will do more tests.
How is FUO treated? —
Often, FUO goes away on its own, and does not need any specific treatment. If tests find a cause for your fever, your doctor or nurse will talk to you about treatment options.
In some cases, medicines can help:
●Medicines like acetaminophen (sample brand name: Tylenol) or ibuprofen (sample brand names: Advil, Motrin) can help bring down a fever. Your doctor or nurse will talk to you about when and how to use these. For example, they might tell you not to take medicine unless you are very uncomfortable. That's because these medicines can make it harder to see the patterns of your fever.
●In some cases, doctors prescribe steroid medicines to see if they treat the fever. They might do this if they suspect that you have a condition that can be treated with steroids.
●Doctors usually do not prescribe antibiotics to treat FUO unless they think that you might have a very serious infection.
What else can I do on my own? —
Stay home, rest, and drink plenty of fluids while you have a fever.
Your doctor or nurse might ask you to keep track of your fevers. This means writing down:
●When you have a fever
●How high your temperature is
●How you took your temperature, for example, using a mouth or forehead thermometer
●Whether you had any other symptoms
●Whether you took medicine to reduce the fever, and how much
They might also ask you to take your temperature at specific times each day and write the numbers down.
Can FUO be prevented? —
Sometimes. Fever is often caused by infection. You can lower your risk of getting an infection if you:
●Wash your hands often (figure 1).
●Stay away from people who are sick.
●Get all of the vaccines your doctor recommends.
●Talk to your doctor or go to a travel clinic if you plan to travel outside of the country. They can suggest ways to avoid getting infections while you are traveling.
When should I call the doctor? —
Call your doctor or nurse if you:
●Have a fever that lasts longer than a few days or keeps coming back
●Start having new symptoms
Patient education: When to worry about a fever in adults (The Basics)
Patient education: Fever in children (The Basics)
Patient education: Fever in babies younger than 3 months (The Basics)
Patient education: How to take a temperature (The Basics)
