Most of the time, fevers lower than 102°F don’t mean that anything is seriously wrong. Your body is probably fighting a common viral or bacterial infection or reacting to, say, a flu shot. It’s rare when this kind of fever indicates a more worrisome condition, like cancer. And once your immune system overwhelms the invading germs, fever goes away.
Yet, some fevers don’t go away. These are called fevers of unknown origin. Uncovering their causes can be a major challenge, although doctors eventually solve 85 percent to 90 percent of them.
Your doctor will diagnose a fever of unknown origin after you’ve had at least a 100.5°F temperature, continually or off and on, for several weeks. Diagnosis routinely begins with a thorough physical exam that usually includes blood and urine samples.
He or she may also order X-rays or an ultrasound, an MRI or a computed tomography (CT) scan. In one special test, called a gallium scan, the patient is injected with a tracer of radioactive gallium. This isotope binds to infected tissue. A day or two later, doctors use a special imaging device and look for “hot spots” where the gallium has gathered. In some cases, a liver or bone marrow biopsy may also be required.
On occasion, these fevers can be linked to organisms that you encounter at work or from places you’ve recently visited. But it’s much more likely that your fever is caused by conditions such as:
- prescriptions that are causing an adverse reaction
- abscesses or infected organ tissue
- arteritis, or inflamed arteries
- endocarditis, an infection of the heart
Depending on the cause, your doctor may prescribe antibiotics for treating a bacterial infection; rest and fluids for viral infections; antifungal or antimicrobial drugs; or over-the-counter fever reducers such as aspirin, acetaminophen or ibuprofen.
See your doctor if you have a persistent fever under 102°F. Avoid self-medicating such a fever—this may make it harder for your doctor to determine what’s wrong. And always get immediate medical attention when a fever goes higher than 102°F—you may be battling a more serious illness.