Continuous Fever, Unknown Fever (Fever of Unknown Origin)

What is a continuous fever?

A continuous fever is a body temperature greater than 37.7C (100F) that continues to persist for 24 hours or more. While there may be fluctuations in the body temperature during this time, the readings do not return to a normal range. An acute fever lasts 4 days or less but may continue for up to 7 days in more severe infections. Chronic fevers are usually recurrent fevers that persist over a long period of time.

An acute fever may be continuous for the short time span that it persists, however, in most cases the fever is episodic. A chronic fever is intermittent with recurrent episodes of an elevated body temperature lasting for a few hours and then easing only to return again within a short period of time. In contrast, a continuous fever is unrelenting.

Continued Fever Readings

  • Normal body temperature = 36.5C (97.7F) to 37.7C (100F)
  • Fever > 37.7 C (100F)
  • Low-grade fever < 38.3C  (101F)
  • Mild fever = 38.3C (101F) * to 40C (104F )
  • High fever > 40C (104F)

* A temperature above 38.3C  (101F) in an infant younger than 3 months is considered high and should be treated as a medical emergency. Refer to Baby Fever.

fever pyrexia

Causes of Continuous Fever


Most fevers are caused by an infection. A continuous fever may be indicative of a severe infection that is progressing and serious complications like sepsis may arise.


A continued fever may also indicate damage to the thermoregulatory center in the hypothalamus (central fever) that is responsible for maintaining the body temperature set point. A continued elevation in body temperature may be a result of impairment of the thermoregulatory functions that are responsible for cooling the body.


Drug-induced fever or drug-induced hypothermia that is continuous is most likely due to the chronic use of drugs for the treatment of medical conditions, abuse of illicit drugs due to addiction or unsupervised use of certain drugs like weight loss medication.

Unknown Fever

Unknown fevers may be due to a number of causes with infections accounting for the majority of the cases. An unknown fever is usually a fever that is non-specific, meaning that apart from the raised body temperature, there are no other signs, symptoms or abnormalities detected upon conducting medical tests and investigations. A fever may be acute or chronic.

child fever

Acute Unknown Fever

In the event of an acute fever, signs and symptoms may develop shortly after the onset of the fever thereby assisting with a diagnosis. The cause may be self limiting and resolve spontaneously or may require medical treatment.

Chronic Unknown Fever

A chronic fever that is due to unknown causes is referred to as a fever of unknown origin (FUO). There are certain criteria that define a fever of unknown origin. Therefore, it is important to consult with a doctor and have the necessary tests before defining a chronic fever as a fever of unknown origin.

  • Chronic Fever
    • Persistent fever
    • Low-grade fever
    • Mild fever
    • High fever

Fever of Unknown Origin

Fever of unknown origin or FUO is a chronic fever for which no cause can be conclusively identified. There are three criteria that define FUO and this includes :

  1. A persistent or recurrent body temperature of 38.3C (101F) or more,
  2. A fever continuing for more than 3 weeks,
  3. No conclusive diagnosis of the cause of the fever despite 3 days of inpatient care or 2 outpatient visits.

Non-specific Fever Tests

A diagnosis of a specific medical condition is made upon the clinical presentation, which are the signs evident to the doctor and symptoms reported by the patient, along with a medical history. Investigations confirm the diagnosis and allows a doctor to assess the severity of the condition.

The presence of a fever with no other symptoms or vague and transient symptoms is referred to as a non-specific fever. In this instance, a diagnosis is difficult to make without further investigation. Some of the investigations that would be considered in the event of a non-specific fever include:

  • White blood cell (WBC) count and/or complete blood count (CBC)
  • Sputum test
  • Throat culture
  • Urinalysis
  • Stool test
  • Lumbar puncture
  • Chest x-ray

Other tests may be considered based on the patient’s medical, travel, occupational, sexual and family history. Recreational habits, drug usage (prescription/narcotic), lifestyle and nutrition as well as vector exposure (animal/insect contact, bites and stings). If no conclusive diagnosis can be reached after the clinical evaluation and investigations, the fever may be referred to as a fever of unknown origin (FUO). A factitious fever is where a patient manipulates the body temperature readings to record higher levels than is actually present and this has to be excluded as a possible cause of a fever of unknown origin.

Causes of a Fever of Unknown Origin

Infectious Causes of a Fever of Unknown Origin

An infection is caused by pathogenic microorganisms. These are the more common cause of a fever of unknown origin.

  • Abscess
    • Especially abdominal, pelvis and dental abscesses.
    • Consider if there is a history of trauma, gynecological procedures, peritonitis, diverticulosis.
  • Cytomegalovirus (CMV)
    • Consider if there is a history of blood transfusion or if HIV-positive.
  • Epstein-Barr virus (EBV)
  • Human Immunodeficiency Virus (HIV)
    • Consider if there is a history of  risky behavior like multiple sexual partners, unprotected sex, IV drug use.
    • Refer to Stages of HIV/AIDS.
  • Endocarditis
    • Inflammation of the inner lining of the heart and heart valves. In an FUO, this is usually due to an infection and is referred to as infective endocarditis.
  • Travel-Related Fevers
    • Consider if there is a history of recent travel.
    • Refer to Traveler’s Fever for a list of causes of a non-specific fever after travel.
  • Osteomyelitis
    • Inflammation of the bone due to an infection.
  • Sinusitis
    • Inflammation of the paransal sinuses due to an infection.
  • Tuberculosis (TB) – especially extrapulmonary TB.
  • Other
    • Fungal infections
    • Parasitic infections

Rheumatological / Autoimmune Diseases

This include inflammatory conditions caused by the body’s immune activity against its own tissues.

  • Adult Still’s disease
  • Giant cell arteritis (temporal arteritis)
  • Inflammatory bowel disease
  • Polyarteritis nodosa
  • Polymyalgia rheumatica
  • Reactive arthritis
  • Rheumatoid arthritis
  • Systemic lupus erythematosus (SLE)

Neoplastic / Malignancy

This includes malignant tumors.

  • Colon cancer
  • Liver cancer (hepatoma)
  • Leukemia and myeloma
  • Lymphoma
  • Renal cell carcinoma
  • Stomach cancer
  • Cancer of the pancreas
  • Metastatic cancer

Miscellaneous (Other Causes)

  • Alcoholic cirrhosis
  • Drug fever
  • Factitious fever – high body temperature due to manipulation by the patient.
  • Deep vein thrombosis (DVT)
  • Pancreatitis (inflamed pancreas)
  • Inflammatory bowel disease
  • Familial Mediterranean Fever (refer to periodic fever syndrome in Chronic Fever)
  • Thyrotoxicosis (hyperthyroidism) and thyroiditis
  • Sarcoidosis


  1. Fever of Unknown Origin. Merck
  2. Fever in Returned Travelers. CDC

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