A high temperature without a clear cause is given the term fever of unknown origin (FUO). A FUO is an intermittent temperature of at least 101°F. It has to have been noted for over three weeks. It also has to have at least a week’s worth of attempts to find the cause.

There are many rare causes of a high temperature. The following list includes just a few of the uncommon causes:

• Unusual infections
  • Extrapulmonary (located elsewhere than in the lungs) tuberculosis
  • Atypical tuberculosis (relatives of the common germ)
  • Tropical diseases in temperate climates/latitudes— malaria , dengue fever , yellow fever , etc
  • Rare organisms—fungi, viruses, uncommon bacteria
• Obscure infections
  • Prostatitis
  • Sinusitis
  • Hidden abscesses
• Collagen vascular (connective tissue, autoimmune) diseases
  • Rheumatoid arthritis
  • Systemic lupus erythematosus

• Inflammatory diseases
  • Sarcoidosis
  • Crohn’s disease (regional ileitis)
• Cancer
  • Lymphoma ( Hodgkin’s and non-Hodgkin’s )
  • Leukemia
  • Kidney cancer
  • Liver cancer
• Drug reactions
  • Antibiotics
  • Epilepsy medications
  • Immune globulin
  • Antipsychotics (eg, Thorazine, Haldol )
  • Antihistamines
• Hereditary metabolic diseases
• Hormone disturbances
  • Hyperthyroidism
• Brain disorders that affect temperature regulation
  • Tumors
  • Strokes

The following factors increase your chance of developing a FUO:

• Foreign travel, especially to developing and tropical countries
• Current medications (both prescription and over-the-counter)
• Cancer or brain tumor
• Collagen vascular disease (an autoimmune disorder of connective tissue)
• HIV/AIDS
• Current or recent hospitalization
• Similar problems in your family

If you experience any of these, do not assume it is due to a FUO. A fever is a very common indication of many problems. They can be both serious and trivial. If you experience any one of them, see your doctor.

• Elevated temperature by thermometer reading
• Sweats
• Chills
• Aching all over

Your doctor will ask about your symptoms and medical history. A physical exam will be done. You can help by taking your temperature several times a day. Your doctor may refer you to a specialist.

The first efforts after the usual evaluation will be to narrow the possibilities. This is done by examining the circumstances under which the fever began. Were you traveling abroad? Were you hospitalized? Is your immune system damaged, for example by AIDS? What medications are you currently taking?

Many different tests may be indicated at some point including:

• Exhaustive studies of blood, urine, and all other bodily products
• Exhaustive imaging studies: x-rays, CT and MRI scans, ultrasound examinations
• Nuclear medicine studies
• Endoscopies (lungs, stomach and intestines, sinuses, etc)
• Biopsies (samples taken by knife or needle) of suspect tissues

There is no treatment for a FUO until the underlying disease is identified. When its cause is discovered, treatment will follow.