Atrium Health Navicent Infectious Disease

Fevers of Unknown Origin

Three generations

A fever is a sign and symptom of over 200 different medical conditions. Known as FUO's, fevers of origin affect children and adults, although the statistics differ significantly: in children, infection is the cause of nearly half of all fevers of unknown origin, while in adults, only 16% of FUO's can be traced back to infections. Additionally, in nearly half of all cases involving adults, no cause can be pinpointed.

A fever is classified as an FUO if the patient has had a fever of at least 101 degrees Fahrenheit (38.3 degrees Celsius) for three weeks or longer. Fevers that reoccur regularly or intermittently without explanation are also classified as fevers of an unknown origin. While most adults can tolerate temporarily raised temperatures with little problem, even a mild fever can cause breathing and heart rate to increase in adults with co-existing conditions such as a heart or lung problem.


There are four main causes of a fever of unknown origin:

  1. Infections such as Lyme disease or tuberculosis
  2. Inflammations such as Crohn's Disease, lupus, or rheumatoid arthritis
  3. Malignancies such as cancers
  4. Miscellaneous causes include hepatitis, hyperthyroidism, drug abuse

While a fever that persists for a long time or reoccurs intermittently is likely to point to a noninfectious cause, infectious causes tend to produce fevers that last for four days or less. Nearly all infectious diseases can cause a fever in an otherwise healthy adult, but the most likely causes for this kind of fever are gastrointestinal infections, respiratory tract infections (upper or lower), skin infections and UTI's (urinary tract infections).

Signs and Symptoms

Aside from the length of time the fever lasts and how high it is, there are other warning signs that may indicate an FUO:

  • Rapid breathing
  • Rapid heartbeat
  • Stiff neck and/or a persistent headache
  • Low blood pressure
  • Flat red spots on the surface of the skin known as petechiae which indicate bleeding under the skin
  • Recent use of autoimmune drugs

Tests and Diagnostic Evaluations

Tests are needed to pinpoint an origin of one particular condition. For example, if the patient has neck and head pain, the doctor may order a spinal tap to see if meningitis is present; if there is a cough accompanying the fever, the chest x-ray will be used to narrow down the suspicion of pneumonia as the cause of the fever.

Risk Factors

Risk factors for getting an FUO include but are not limited to, exposure through contact with a person or insect carrying an infectious disease, which is why the patient will be asked about any recent travels to areas of the world where infectious diseases are common. The person's age and health status are also factored in, use of certain medical procedures and some occupations. For instance, workers in meat packing plants are at a much higher risk for developing a bacterial infection called brucellosis than the rest of the population.


A physician will determine if a fever of unknown origin is present by administering a physical evaluation, a series of questions regarding the patient's health history and recent possible exposures, and sometimes a urine test or chest x-ray. A complete blood count will show if there has been an increase in the white blood cell count since that indicates that an infection is causing the FUO.

Among the questions the doctor may ask are:

  • Does the patient have any conditions such as diabetes, cancer, HIV or a heart valve disorder that could predispose him or her towards an infection?
  • Has the patient been in contact with anyone who already has an infection?
  • Does the patient have any autoimmune diseases such as gout, lupus or cancer which predispose him or her to infection?
  • Does the patient inject illegal drugs?
  • Has the patient had recent exposure to animals including pets?
  • Does your family history include any possible hereditary causes for a fever?


Since fever is not an illness itself - it's a sign that the body is trying to defend itself against infection- it isn't strictly necessary to treat the fever alone, but successfully lowering the temperature will make almost any patient feel more comfortable. Patients suffering from dementia or heart or lung problems are considered to be at a higher risk of developing dangerous complications from an FUO, so they so they should be treated promptly if a fever is present.

Treatments for FUO's range from no treatment at all (since approximately 20% of fevers of unknown origin resolve themselves without a diagnosis) to simple drugs to lower the body temperature; the most effective of these antipyretics such as acetaminophen, ibuprofen, and naproxen.

The old idea of putting patients with high fevers into cold baths is not the current recommendation. However, if the fever is extremely high cooling blankets and a room temperature mist may be used to increase the patient's comfort. Do not use alcohol under any circumstances to sponge-bathe a patient because it is absorbed through the skin and may produce harmful reactions.