Transmission pathways in emerging infections: linking clinical presentation, epidemiology, and outbreak potential in humans.

BACKGROUND

Emerging infectious diseases are shaped by ecological disruption, climate change, urbanisation, and demographic change. Although transmission routes are frequently characterized in isolation, they are fundamental determinants of how pathogens establish infection, spread through populations, and generate outbreaks.

OBJECTIVES

To examine how transmission pathways influence clinical presentation, epidemiological behaviour, outbreak dynamics, and preparedness in emerging human infections, and to propose an operational framework supporting early risk assessment and public health response.

SOURCES: This narrative review was informed by a targeted literature search of PubMed/MEDLINE, Scopus, and Web of Science for literature published between January 2010 and December 2025, supplemented by reference screening of relevant reviews and seminal articles published before this period.

CONTENT: Transmission pathways shape disease behaviour through their effects on the portal of entry, early replication site, shedding dynamics, timing of infectiousness, and opportunities for amplification. These mechanisms influence symptom patterns, dissemination, asymptomatic infection, and transmissibility, and generate characteristic outbreak patterns, including rapid respiratory spread, contamination-driven outbreaks, ecologically constrained focal transmission, and network-based clustering. Their effects are further modified by host susceptibility, mobility, infrastructure, climate-sensitive ecological change, and social conditions, which together determine whether infections remain focal, re-emerge in vulnerable settings, or expand more widely.

IMPLICATIONS: An operational framework provides a mechanistic basis for aligning surveillance, diagnostics, infection prevention, modelling, vaccination, and public health response with dominant transmission pathways. Applied prospectively, such a framework may be especially useful in the early stages of emergence, when transmission patterns are often recognized before the pathogen is fully characterized, thereby helping identify high-risk pathways, anticipate amplification settings, and guide more timely and context-specific preparedness for future emerging infectious threats.