A fever that comes and goes for days is usually nothing more than a viral illness running its course. But a fever that persists for three weeks or more without explanation — despite basic evaluation — is an entirely different clinical situation. This condition has a specific name: Fever of Unknown Origin, or FUO. And it is one of the most challenging — and fascinating — diagnostic puzzles in all of medicine.
At Florida Infectious Disease Care in Fort Myers, our board-certified specialists are experienced in the systematic, methodical investigation required to identify the cause of fever of unknown origin. If you or someone you care about has had an unexplained fever lasting weeks, this guide explains exactly what the investigation involves and why a specialist is the right physician to lead it.
What Is Fever of Unknown Origin?
Fever of Unknown Origin was formally defined in a landmark 1961 paper by Petersdorf and Beeson as a fever exceeding 101°F (38.3°C) on multiple occasions, lasting more than three weeks, with no diagnosis reached after one week of thorough inpatient investigation. Over time, the definition has been refined for the outpatient setting, but the core concept remains: a prolonged fever that resists initial diagnostic efforts.
FUO is not a diagnosis itself — it is a clinical challenge that demands a structured, evidence-based investigative approach. According to a comprehensive review in the New England Journal of Medicine, infectious causes account for approximately 20 to 40 percent of FUO cases, non-infectious inflammatory conditions account for another 20 to 30 percent, malignancies account for 15 to 20 percent, and in approximately 50 percent of cases, no cause is ever found despite thorough investigation.
Common Infectious Causes of Fever of Unknown Origin in Florida
Because Florida’s geography, climate, and demographics create a unique infectious disease landscape, certain causes of FUO deserve specific attention in Southwest Florida patients:
Endocarditis
Infective endocarditis — infection of the heart valves — is a classic cause of prolonged fever that can smolder for weeks before being diagnosed. Subacute endocarditis caused by viridans streptococci or Enterococcus presents insidiously with low-grade fever, fatigue, and joint pain that are easily mistaken for other conditions. Multiple blood cultures collected before any antibiotics remain the cornerstone of diagnosis.
Tuberculosis
While tuberculosis rates in the United States are low, Florida’s large immigrant population from regions with higher TB prevalence makes it a relevant consideration in FUO evaluation. Extrapulmonary tuberculosis — affecting the lymph nodes, liver, bones, or kidneys rather than the lungs — is a particularly challenging cause of FUO because it may not produce the classic respiratory symptoms that prompt TB testing. The Florida Department of Health monitors TB rates closely across the state.
Intra-abdominal or Pelvic Abscesses
A collection of infected fluid deep within the abdomen or pelvis can produce prolonged fever with minimal localizing symptoms. These abscesses frequently develop after abdominal surgery, appendicitis, diverticulitis, or Crohn’s disease flares. CT imaging of the abdomen and pelvis is a critical component of the FUO workup in most patients.
Tick-Borne and Vector-Borne Infections
Southwest Florida’s outdoor environment and tick population make vector-borne infections a relevant FUO consideration. Ehrlichiosis, anaplasmosis, and rickettsia infections can all cause prolonged fever, often with leukopenia and elevated liver enzymes. A thorough exposure history — including tick bites, outdoor activities, and travel — is essential in every FUO evaluation.
Viral Infections
Certain viral infections produce prolonged febrile illness. Epstein-Barr virus (EBV), cytomegalovirus (CMV), and HIV itself can all cause weeks of fever, particularly in patients who are immunocompromised or experiencing primary infection. HIV-related fever of unknown origin — in patients who are undiagnosed or who have advanced immunosuppression — is a specific and important consideration in Florida’s high-HIV-burden environment.
Non-Infectious Causes of Fever of Unknown Origin
Not all cases of FUO are infectious. A significant proportion of cases are caused by inflammatory, rheumatological, or malignant conditions:
- Adult-onset Still’s disease — an autoinflammatory condition causing spiking fevers, rash, and joint pain
- Systemic lupus erythematosus (SLE) — fever is a common manifestation of lupus flares
- Giant cell arteritis and polymyalgia rheumatica — particularly in patients over 50, with headache and jaw claudication as associated features
- Lymphoma — Hodgkin’s lymphoma classically causes Pel-Ebstein fever, a pattern of recurring fever separated by afebrile intervals
- Solid organ malignancies — renal cell carcinoma, hepatocellular carcinoma, and colon cancer can all cause fever as a paraneoplastic phenomenon
- Drug fever — a surprisingly common and frequently overlooked cause, where a medication causes fever through immune-mediated or idiosyncratic mechanisms
The FUO Investigation: How Infectious Disease Specialists Approach It
The investigation of FUO requires a methodical, step-by-step approach guided by the history, examination, and evolving laboratory results rather than shotgun testing. At Florida Infectious Disease Care, our approach includes:
- Detailed history: travel, animal exposures, occupational risks, sexual history, medication list, family history, and the precise pattern and character of the fever
- Thorough physical examination repeated over time — physical findings can evolve and provide critical diagnostic clues
- First-tier laboratory testing: multiple blood cultures, complete blood count, comprehensive metabolic panel, inflammatory markers (CRP and ESR), urinalysis, urine culture, and serological tests for infectious and inflammatory conditions
- Imaging: chest X-ray and CT of the chest, abdomen, and pelvis to identify occult abscesses, lymphadenopathy, or masses
- Second-tier testing based on findings: bone marrow biopsy, liver biopsy, PET-CT scanning, echocardiography, lumbar puncture when indicated
The goal is not to order every test simultaneously but to follow a logical sequence in which each result guides the next step. This is where infectious disease specialist expertise is irreplaceable — pattern recognition developed through years of investigating complex febrile illness.
When to Seek Evaluation for an Unexplained Fever in Fort Myers
You should seek evaluation at Florida Infectious Disease Care if you have a fever exceeding 101°F that has been present on multiple occasions over a period of more than two to three weeks without an established cause. Additional urgent triggers include a fever accompanied by night sweats and unexplained weight loss, a fever in an immunocompromised patient, or a fever that is accompanied by focal symptoms such as back pain, joint swelling, or neurological changes.
Frequently Asked Questions
Q: Can stress or anxiety cause a prolonged fever?
True fever — a core body temperature above 101°F — is not caused by psychological stress alone. However, a condition called psychogenic fever, associated with extreme emotional distress, can cause elevated body temperature in rare cases. It is important to distinguish true fever from elevated temperature readings caused by technical factors — such as measuring temperature after exercise or in a warm environment. Any persistent true fever requires medical evaluation.
Q: How long will it take to find the cause of my FUO?
This varies enormously depending on the underlying condition. Some causes — such as endocarditis — are identified relatively quickly once blood cultures are incubated. Others — such as adult-onset Still’s disease or certain lymphomas — may take weeks of systematic investigation to diagnose. In approximately 50 percent of FUO cases, no cause is ever identified despite thorough investigation, and the fever resolves spontaneously. Your specialist will keep you fully informed at every step of the process.
Q: Should I take antibiotics while waiting for a diagnosis?
In most cases, empirical antibiotics should be avoided until a probable cause has been identified. Antibiotics started without a clear target can suppress bacterial cultures — preventing identification of the causative organism — and may mask clinical signs that would otherwise guide the diagnosis. There are specific high-suspicion situations where empirical therapy is warranted, and your specialist will make that determination based on your clinical picture.
Q: Can a fever of unknown origin be serious even if I feel relatively well?
Yes. Some of the most significant underlying causes of FUO — including lymphoma, endocarditis, and early tuberculosis — can produce prolonged fever in patients who feel surprisingly functional. Feeling relatively well does not exclude a serious underlying condition. Persistent fever is always a signal that something needs investigation, regardless of how the patient otherwise feels.
Ready to Get the Expert Care You Deserve?
At Florida Infectious Disease Care, our board-certified specialists bring decades of combined expertise to every patient encounter — with compassion, precision, and complete confidentiality. Whether you are navigating a new diagnosis, a recurring infection, or a prevention decision, we are the right team for you. Call us today at 239-245-8223 or visit us at 14192 Metropolis Ave, Fort Myers, FL 33912.
Book your appointment at floridaidcare.com/make-an-appointment or find us on Google Maps. Expert infectious disease care is closer than you think — and your health cannot wait.