Fever is just one of the number of symptoms that accompany most infections such as Covid and  influenza. When doctors can’t find a diagnosis for the fever, and it lasts for a few weeks, however, it is called fever of unknown origin, or FUO.

There are a bewildering number of illnesses that produce fever, and the mixture of these illnesses is different depending on geographic location, the type of hospital, and socioeconomic conditions.

Just like weight loss of unknown origin, or abdominal discomfort of unknown origin, fever without obvious cause is quite possibly be due to cancer in affluent America, and if you go in early you might have better outcomes with your treatment.

Fever has been known since earliest times, and was often considered a diagnosis on its own. In the past, the great majority of the fevers were infectious, and the outcome grave. In the mid 20th century, when I went to medical school, fevers were still mostly infectious. Antibiotics were the magic bullet, and were unfortunately overused. In underdeveloped countries, infections are still the most common cause, but in the developed world difficult to treat viral infections, autoimmune conditions, and cancer have been gaining in prominence.

When fever becomes excessive, and medication is needed, NSAIDS may be used, and works better on fever from infection than on fever from cancer. The take-home message for me is that if you use Naprosyn for a persistent fever, and isn’t effective, you might notify the doctor.

The motivation for me writing this article came from a very good posting in the New England Journal of medicine. They used a little humor, stating that modern FUO might be called “fever of too many origins”, what with all the indwelling catheters, implanted medical devices, shunts and long hospital stays. There is a separate category made for fever acquired in the hospital.

In people with AIDS, the evaluation is different depending on whether or not they are on treatment.

Tuberculosis is still a very common cause of fever.

Drugs are becoming increasingly responsible for troublesome fevers. In the early days of antibacterial therapy, sulfa  was the only drug available, frequently caused fever.  Now, sulfa is less used, and the penicillin derivatives are more common causes of fever.

If you have a fever, and have been traveling recently, be sure to tell the doctor. Your fever might be due to a tropical parasite such as malaria, particularly if you’ve been to West Africa.

Fever is an evolutionarily conserved body defense reaction and helps a person recover from an infection. The normal body temperature cycles according to the time of day; it is lowest first thing in the morning, and is higher later in the afternoon. The average body temperature used to be 37°C, or 98.6 F., but has been declining in recent decades, and is now about 36.5°C or 97.6°F. The use of electronic thermometers has cut down the amount of time needed to assess the body temperature, but added variability. I still prefer the old-fashioned thermometer.

Taking your temperature by whatever means you have available still remains a good idea when you don’t feel well.

—Dr. C.