A comprehensive and meticulous history (i.e. illness of family members, recent visit to the tropics, medication), repeated physical examination (i.e. skin rash, eschar, lymphadenopathy, heart murmur) and myriad laboratory tests (serological, blood culture, immunological) are the cornerstone of finding the cause. Other investigations may be needed. Ultrasound may show cholelithiasis, echocardiography may be needed in suspected endocarditis and a CT-scan may show infection or malignancy of internal organs. Another technique is Gallium-67 scanning which seems to visualize chronic infections more effectively. Invasive techniques (biopsy and laparotomy for pathological and bacteriological examination) may be required before a definite diagnosis is possible. Positron emission tomography using radioactively labelled fluorodeoxyglucose (FDG) has been reported to have a sensitivity of 84% and a specificity of 86% for localizing the source of<unk> . Despite all this, diagnosis may only be suggested by the therapy chosen. When a patient recovers after discontinuing medication it likely was drug<unk> , when antibiotics or antimycotics work it probably was infection. Empirical therapeutic trials should be used in those patients in which other techniques have failed. Definition There is no universal agreement with regards to time criteria or other diagnostic criteria to diagnose a<unk>  and various definitions have been used. In 1961 Petersdorf and Beeson suggested the following criteria: *<unk>  higher than 38.3 °C (101 °F) on<unk> al occasions * Persisting without diagnosis for at least 3 weeks * At least 1 week's investigation in hospital A new definition which includes the outpatient setting (which reflects current medical practice) is broader, stipulating: * 3 outpatient visits or * 3 days in the hospital without elucidation of a cause or * 1 week of "intelligent and invasive" ambulatory investigation. Presently<unk>  cases are codified in four subclasses. Classic<unk>  This refers to the<unk> al classification by Petersdorf and Beeson. Studies show there are five categories of conditions: * infections (e.g. abscesses, endocarditis, tuberculosis, and complicated urinary tract infections), * neoplasms (e.g. lymphomas, leukaemias), * connective tissue diseases (e.g. temporal arteritis and polymyalgia rheumatica, Still's disease, systemic lupus erythematosus, and rheumatoid arthritis), * miscellaneous disorders (e.g. alcoholic hepatitis, granulomatous conditions), and * undiagnosed conditions. Nosocomial Nosocomial<unk>  refers to pyrexia in patients that have been admitted to hospital for at least 24 hours. This is commonly related to hospital-associated factors such as surgery, use of a urinary catheter, intravascular devices (i.e. "drip", pulmonary artery catheter), drugs (antibiotic-induced Clostridium difficile colitis, drug<unk> , and/or immobilization (decubitus ulcers). Sinusitis in the intensive care unit is associated with nasogastric and orotracheal tubes. Other conditions that should be considered are deep-vein thrombophlebitis, pulmonary embolism, transfusion reactions, acalculous cholecystitis, thyroiditis, alcohol/drug withdrawal, adrenal insufficiency, and pancreatitis. Immune-deficient Immunodeficiency can be seen in patients receiving chemotherapy or in hematologic malignancies.<unk>  is concomitant with neutropenia (neutrophil <500/uL) or impaired cell-mediated immunity. The lack of immune response masks a potentially dangerous course. Infection is the most common cause. Human immunodeficiency virus (HIV)-associated HIV-infected patients are a subgroup of the immunodeficient<unk> , and frequently have<unk> . The primary phase shows<unk>  since it has a mononucleosis-like illness. In advanced stages of infection<unk>  mostly is the result of a superimposed infections.
Fever of unknown origin
Q fever
Relapsing fever
Pontiac fever
Enterotoxemia
Cat Scratch Fever
Morning sickness
Streptococcal pharyngitis