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What Causes a Chronic Fever?

Tricia Christensen
Updated Mar 03, 2024
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Chronic fever, often also called fever of unknown origin (FUO), is a fever that keeps recurring or doesn’t relent and doesn’t have an immediate explainable cause, like an obvious viral or bacterial infection. Such a condition may have numerous causes, and doctors advise people not to ignore this symptom. The degree to which FUO is serious depends on its source, but since it may suggest serious illness, a doctor’s visit is warranted.

Many times doctors look first to one of the obvious causes of continuous fever: infection in the body. This could be relatively hidden and might be due to an abscess somewhere in the body or something like infection of the urinary tract. Cat scratch fever, a bacterial infection, sometimes develops symptoms like continuous fever and swollen lymph glands, especially in children, and can persist for months. Doctors are frequently able to verify bacterial infection with blood tests or body scans, and by treatment, they can eliminate the fever.

Certain types of viral infections can also cause chronic fever. Patients who have undiagnosed chronic fatigue syndrome, might have FUO. HIV might also be manifested with a chronic or recurring fever.

There are instances where continuous fever is evidence of diseases that suppress the immune system. Conditions like lupus, juvenile and adult rheumatoid arthritis, and HIV/AIDS result in a fairly constant fever. In these cases, bacterial infection doesn’t need to be actively present, but the body produces fever in response to what it thinks is a constant assault on its immune system. Alternately, illnesses like sarcoidosis may cause the body to respond with a chronic low grade fever.

More serious are certain cancers that may result in chronic fever. Diseases like lymphoma have an effect on the immune system and FUO could be an early symptom. Other forms of cancer may also produce fever.

Sometimes the body responds to injury by developing a fever. If a bone breaks or tissue damage occurs, the body could produce fever as a misdirected healing response and this usually continues until healing is advanced. Alternately, the origin could be another unrelated medical condition like heart disease, occasionally blood pressure conditions, or infections in the heart like bacterial endocarditis, which compromise its function.

Imbalance in some of the body’s hormones may also result in chronic fever. When people have conditions like hyperthyroidism, where they produce too much thyroid hormone, they may run fevers. This condition is easy to verify with blood testing.

A generally more benign cause of a chronic fever is reaction to certain medications. Some people can continue to take medications but might run a slight fever when they use certain kinds. For other patients, fever from medicines might represent severe issues. If fever is burdensome or problematic, medications could be switched or discontinued.

The Health Board is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Tricia Christensen
By Tricia Christensen
With a Literature degree from Sonoma State University and years of experience as a The Health Board contributor, Tricia Christensen is based in Northern California and brings a wealth of knowledge and passion to her writing. Her wide-ranging interests include reading, writing, medicine, art, film, history, politics, ethics, and religion, all of which she incorporates into her informative articles. Tricia is currently working on her first novel.
Discussion Comments
By fify — On Oct 15, 2013

@ddljohn-- Did you have swollen lymph glands, muscle aches and pains and fatigue as well?

My aunt had all these symptoms. After seeing three different doctors, they finally figured it out. It was cat scratch fever!

By ddljohn — On Oct 14, 2013

@MikeMason-- Fever of unknown origin is when doctors cannot identify a cause at that time. So until the cause is found, it's referred to as an FUO.

I had an FUO a few months ago. It lasted for a few weeks. I had all sorts of diagnostic tests done and they couldn't find anything. I was treated with antibiotics and fever reducers anyway and now I'm fine. But it was scary for a while.

By stoneMason — On Oct 13, 2013

I don't think that there is such a thing as fever of unknown origin. There is always a cause, but sometimes it takes a while to figure it out.

Fever is almost always a sign of an infection in the body. Our immune system increases our body's temperature on purpose to kill bacteria and or viruses. When it's chronic, it means that there is an ongoing infection that the immune system is having a hard time fighting.

I had chronic fatigue and fever last year which lasted months. I resisted seeing a doctor at first because I didn't have good health insurance but when the fever persisted, I had to go. It turned out to be mononucleosis and I had to take antibiotics. Even after my treatment was over, the fever kept coming back. I had fever on and off for three or four months. It eventually went away.

Tricia Christensen
Tricia Christensen
With a Literature degree from Sonoma State University and years of experience as a The Health Board contributor, Tricia...
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