Why don't I get a fever

Fever of unknown cause (FUO)

Fever can occur as part of a harmless cold, or it can be a sign of serious illness. In some cases, the doctor cannot initially determine the cause of the fever despite a thorough examination. In the case of fever without an identifiable cause of more than 3 weeks with temperatures above 38.3 ° C and unsuccessful standard examinations, one speaks in technical terms of the so-called "fever of unknown origin" FUO).

Doctors distinguish 4 types of fever of unknown cause:

  • Classic FUO
  • Nosocomial FUO
  • FUO in people with a weakened immune system due to a low level of white blood cells (neutropenic FUO)
  • FUO in people with HIV

The most common causes of FUO are undetected infections, tumor diseases or rheumatic diseases or autoimmune diseases. A possible cause of infection is, for example, tuberculosis or an abscess in an atypical location. Such collections of pus can result from ruptured appendicitis or diverticulitis, or from malignant tumors or injuries.

Especially in older people, a fever of unknown cause is often caused by malignant tumors that are difficult to determine and therefore remain undetected for a long time, such as chronic leukemia, lymphoma, kidney, colon or pancreatic cancer or daughter tumors (metastases ) of tumors. Autoimmune diseases can trigger a fever, e.g. Still syndrome, systemic lupus erythematosus or inflammation of arterial blood vessels (e.g. temporal arteritis).

Medicines can also explain a fever of unknown cause if the body is hypersensitive to certain active substances, such as those contained in water that is drained (diuretics), pain relievers, antibiotics, anti-allergic drugs (antihistamines) or sedatives or sleeping pills. These "hypersensitivity reactions" are then expressed as a fever. Here you will find some examples of active substances that can trigger a fever. In principle, however, any medication can be considered for a hypersensitivity reaction with a fever.

Classic FUO

The classic Fever of unknown cause usually lasts longer than 3 weeks. It is caused by infections, malignant tumors or diseases of the connective tissue (rheumatic diseases, collagenoses) or blood vessels (vasculitis).

Nosocomial FUO

As nosocomiales Fever of unknown origin is a fever that occurs after a patient is admitted to the hospital who has not had a fever at the time of admission. Nosocomial FUO is often caused by a pulmonary embolism, an infection of the intestine with the bacterium Clostridium difficile or a phlebitis with the bacteria spreading through the bloodstream. Medicines can also trigger such a fever. In patients fed through a nasogastric tube, inflammation of the sinuses (sinusitis) may be the cause of the fever.

Neutropenic FUO

Neutropenic fever of unknown origin occurs in people who have fewer than 500 so-called neutrophils (white blood cells) per cubic millimeter of blood. Most of the time, the fever in these patients is caused by an infection with bacteria. Fungal infections (e.g. aspergillosis or Kandida infection) or viruses (e.g. herpes viruses) can also cause neutropenic FUO. Typical of these complications is that no infected organs are often observed. On the other hand, bacteria are often found in the blood, but it is not possible to identify the route by which the pathogens entered the body.

FUO in people with HIV

An acute infection with the HI virus can be accompanied by a fever. However, a fever can also occur during the course of the HIV disease. If the cause of the fever cannot be identified in an outpatient HIV patient after 4 weeks and in an inpatient HIV patient after 3 days, doctors speak of an HIV-associated FUO. It is usually caused by pathogens that do not cause infection in healthy people, for example cytomegalovirus, Pneumocystis jeroveci or Mycobacterium avium-intracellulare. Other causes of HIV-associated FUO are tumors that occur very rarely in healthy people, such as Kaposi's sarcoma or cancer of the lymph glands (lymphoma). Medicines can also trigger this form of fever.