Acute febrile illness (AFI) is among the most frequent reasons for seeking care in tropical and subtropical settings, yet its causes overlap clinically and often remain unconfirmed. This paper synthesizes recent and foundational evidence on the clinical characteristics, etiological spectrum, and outcomes of AFI, with emphasis on prospective observational studies from India and meta-analyses from low- and middle-income countries. Sources were located through targeted searches and prioritized for 2023–2026 publications, supplemented by earlier hospital cohorts that report patient outcomes. Across the literature, dengue, scrub typhus, enteric fever, malaria, and leptospirosis recur as leading causes, but their relative weight shifts with region, season, and care setting. A substantial share of febrile patients, from roughly one-third to nearly two-thirds depending on setting, never receive a confirmed diagnosis. Indian hospital cohorts report mortality in the low single digits, with greater need for organ support in scrub typhus, leptospirosis, and malaria. Major barriers include limited diagnostic capacity, imperfect rapid tests, cross-reactivity, discretionary testing, and non-standardized study methods. The paper argues for locally informed diagnostic algorithms, confirmatory testing, standardized prospective protocols, and integrated surveillance.