Acute Febrile Illness Definition
Acute febrile illness (AFI) is defined as fever with temperature ≥38.0°C (100.4°F) of less than 14 days duration, representing a syndrome caused by diverse infectious pathogens including bacterial, viral, protozoal, and rickettsial organisms. 1, 2
Core Temperature Thresholds
The definition varies slightly based on clinical context and measurement method:
Rectal temperature >38°C (>100.4°F) remains the gold standard for accuracy and is the primary definition used by the American College of Physicians 1, 3
Alternative threshold: temperature ≥38.3°C (101°F) as a single measurement, or ≥38.0°C (100.4°F) sustained over 1 hour, per National Comprehensive Cancer Network recommendations 1
In specific surveillance contexts (such as hantavirus monitoring), fever may be defined as >101.0°F (>38.3°C) per CDC criteria 1
Duration Component
AFI specifically refers to acute onset fever with duration <1 week (or <14 days in some surveillance definitions) 1, 2
This distinguishes AFI from fever of unknown origin (FUO), which typically requires ≥5 days of fever without identified source 1, 3
Clinical Presentation Subtypes
Fever without localizing signs is a specific AFI subtype defined as acute onset fever (<1 week duration) with absence of localizing signs on physical examination 1, 3
AFI encompasses presentations with or without accompanying symptoms, including classic syndromes like dengue fever, ehrlichiosis, and other tickborne febrile illnesses 1
Etiologic Spectrum
The AFI syndrome reflects diverse infectious causes including bacterial, viral, protozoal, and rickettsial pathogens 1
In endemic regions, malaria (Plasmodium) is the most commonly detected pathogen (35.8% as single pathogen), though non-malarial causes are increasingly recognized 2
Arboviruses (chikungunya, dengue, zika), leptospirosis, rickettsioses, and respiratory viruses are important but often underrecognized etiologies 2, 4, 5