Febrile Illness with Uncertain Cause (Adult)

You have a fever, but the cause is unknown. A fever is the body's natural reaction to an illness such as infection due to a virus or bacteria. Sometimes other conditions such as cancer or immune diseases can cause fever. This might be more likely if your fever has lasted for more than a week or 2. In most cases, the higher temperature itself isn't harmful. It actually helps the body fight infections. A fever doesn't need to be treated unless you feel very uncomfortable.

Sometimes a fever can be an early sign of a more serious infection. So follow up if your condition gets worse.

Home care

Unless given other instructions by your healthcare provider, follow these guidelines when caring for yourself at home.

General care

  • If your symptoms are not severe, rest at home for the first 2 to 3 days. When you are active again, don't let yourself get too tired.

  • For your overall health, don't smoke. Also stay away from secondhand smoke.

  • You may not feel like eating too much. So a light diet is fine. Stay hydrated by drinking 6 to 8 glasses of fluids per day. This includes water, soft drinks, sports drinks, juices, tea, or soup. If you have congestion, extra fluids will help loosen secretions in the nose and lungs.


  • You can take acetaminophen or ibuprofen for pain or to lower your temperature, unless you were given a different medicine to use. If you have chronic liver or kidney disease or have ever had a stomach ulcer or gastrointestinal bleeding, talk with your healthcare provider before using these medicines. Also talk with your provider if you are taking medicine to prevent blood clots. Don't give aspirin to anyone younger than age 19 unless directed by the provider. It may cause a serious illness called Reye syndrome. This most often affects the brain and liver.

  • If you were given antibiotics for an infection, take them until they are used up, or your healthcare provider tells you to stop. It's important to finish the antibiotics even if you feel better. This is to make sure the infection has cleared. Antibiotics are not often given for a viral infection or a fever with an unknown cause.

  • Over-the-counter medicines will not shorten the length of the illness. But they may be help with symptoms. These include cough, sore throat, or nasal and sinus congestion. Ask your pharmacist for product suggestions. Don't use decongestants if you have high blood pressure.

Follow-up care

Follow up with your healthcare provider, or as advised.

  • If a culture or other lab tests were done, you will be told if your treatment needs to be changed. Call your healthcare provider as directed for the results.

  • If X-rays, a CT scan, MRI, or an ultrasound were done, a specialist will review them. You will be told of any findings that may affect your care. Call your healthcare provider as directed for the results.

Call 911

Call 911if any of these occur:

  • Trouble breathing or swallowing, or wheezing

  • Chest pain

  • Confusion

  • Extreme drowsiness or trouble waking up

  • Fainting or loss of consciousness

  • Fast heart rate

  • Low blood pressure

  • Vomiting blood, or large amounts of blood in stool

  • Seizure

When to get medical advice

Call your healthcare provider right away if any of these occur:

  • Cough with lots of colored sputum (mucus) or blood in your sputum

  • Severe headache

  • Face, neck, throat, or ear pain

  • Feeling drowsy

  • Belly pain

  • Repeated vomiting or diarrhea; bloody diarrhea

  • Joint pain or a new rash

  • Burning when urinating

  • Fever of 100.4°F (38°C) or higher, or as directed by your provider

  • Shaking chills

  • Feeling weak or dizzy

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