Fever of Unknown Origin

Fever of unknown origin (FUO) is a condition in which an individual experiences persistent fever without a clear underlying cause despite thorough diagnostic evaluation. Typically, a fever is defined as a body temperature higher than 100.4°F (38°C). A fever is considered a FUO when it persists for more than three weeks, with no identified source of infection, malignancy, or other underlying condition.


The cause of FUO can be challenging to diagnose, and it often requires extensive testing. Infectious diseases account for 20-40% of FUOs, with tuberculosis, endocarditis, and HIV being common culprits. Other causes include autoimmune diseases, malignancies, and drug reactions. In rare cases, FUOs may result from less common infections, such as parasitic infections, fungal infections, and tick-borne diseases.


Apart from fever, patients with FUO may experience other symptoms, such as weight loss, fatigue, night sweats, and chills. The fever may be accompanied by other symptoms, such as headache, joint pain, and muscle pain.


FUO diagnosis involves a thorough medical history, physical examination, and laboratory tests. The first step in the diagnostic process is to rule out the most common causes of fever, such as infections, malignancies, and autoimmune diseases. Diagnostic tests may include blood tests, imaging studies, and tissue biopsies. The diagnostic workup may require more invasive procedures, such as bone marrow biopsy or exploratory laparotomy.


The treatment of FUO depends on the underlying cause. In most cases, treating the underlying condition can cure the fever. However, if the cause remains unknown, treating the fever may involve using antipyretics such as acetaminophen or ibuprofen to reduce fever symptoms.


Since FUO can arise from various conditions, prevention is not always possible. However, certain practices can reduce the risk of infections that may lead to FUO, such as good hygiene, avoiding close contact with individuals who are ill, and staying up to date on vaccinations.

When to see a doctor:

If an individual experiences fever that persists for more than three weeks, they should consult with their healthcare provider. They may need to undergo diagnostic testing to identify the cause of the fever. If fever is accompanied by other symptoms such as weight loss, fatigue, or night sweats, it is essential to see a doctor as soon as possible.


Fever of unknown origin is a condition that can be challenging to diagnose and treat. It is essential to see a healthcare provider if one experiences persistent fever. While FUO may be caused by several underlying conditions, many of these conditions can be treated effectively with early detection and appropriate treatment.