Emerging viral pathogens are viruses that have newly appeared in a population or whose incidence or geographic range is rapidly expanding. Over the past several decades, the frequency of emerging infectious disease events has increased dramatically, driven by ecological disruption, climate change, global travel, and intensification of livestock production. Understanding the factors that drive viral emergence is essential for pandemic preparedness and prevention.
Drivers of Viral Emergence
The emergence of novel viral pathogens is driven by a complex interplay of ecological, environmental, and anthropogenic factors. Land-use change, including deforestation, agricultural expansion, and urbanization, brings humans into increased contact with wildlife reservoirs and their viruses, disrupting the ecological barriers that historically limited spillover. Climate change alters the geographic distribution of vector-borne viruses such as dengue, chikungunya, and Zika, expanding their range into previously temperate regions as mosquito vectors (Aedes aegypti and Aedes albopictus) extend their habitat. Wildlife trade and bushmeat hunting create direct pathways for pathogen transmission from animals to humans. Global air travel enables an infected individual to travel from a remote village to a major city in less than 24 hours, facilitating rapid international spread, as demonstrated by the 2014–2016 West African Ebola epidemic and the COVID-19 pandemic. Intensification of livestock production, particularly the concentration of genetically uniform animals in confined facilities, creates conditions for pathogen amplification and the emergence of novel strains.
Zoonotic Spillover and the One Health Concept
Most emerging infectious diseases are zoonotic, originating in animal reservoirs before crossing the species barrier to infect humans. The spillover process involves a sequence of steps: the pathogen must be present in the reservoir host, be shed in sufficient quantities, survive in the environment, contact a susceptible human, and successfully establish infection. Each step is modulated by ecological, behavioral, and immunological factors that determine the probability of emergence. The One Health approach recognizes that human health, animal health, and environmental health are interconnected and advocates for collaborative, cross-disciplinary efforts to monitor and control emerging infectious diseases at the human-animal-environment interface. Surveillance of wildlife and livestock for novel viruses with pandemic potential, combined with risk assessment based on viral traits such as receptor binding, replication capacity, and immune evasion, enables early warning and rapid response.
Coronaviruses
Coronaviruses are enveloped positive-sense RNA viruses with the largest known RNA genomes (approximately 30 kb) that have caused three major outbreaks in the past two decades: severe acute respiratory syndrome (SARS-CoV) in 2002–2003, Middle East respiratory syndrome (MERS-CoV) since 2012, and SARS-CoV-2 causing the COVID-19 pandemic beginning in 2019. Coronaviruses infect a wide range of animal hosts including bats, which serve as the likely ancestral reservoir for both SARS-CoV and SARS-CoV-2, often through an intermediate host such as civet cats (SARS-CoV) or potentially pangolins (SARS-CoV-2). The coronavirus spike protein, responsible for receptor binding and membrane fusion, is the primary determinant of host range and tissue tropism. SARS-CoV-2 uses angiotensin-converting enzyme 2 (ACE2) as its entry receptor, with the spike protein receptor-binding domain (RBD) binding ACE2 with high affinity. The emergence of SARS-CoV-2 variants of concern, including Alpha, Beta, Gamma, Delta, and Omicron, has demonstrated the capacity of RNA viruses to evolve rapidly in humans, acquiring mutations that enhance transmissibility, antibody evasion, and in some cases, change disease severity.
Filoviruses
Filoviruses, including Ebola virus (EBOV) and Marburg virus (MARV), cause severe hemorrhagic fevers with high case fatality rates. These enveloped negative-sense RNA viruses have filamentous virions characteristic of the family Filoviridae. Ebola virus disease (EVD) has an average case fatality rate of approximately 50%, ranging from 25% to 90% in different outbreaks. Filoviruses replicate in a wide range of cell types, with macrophages, dendritic cells, and endothelial cells being primary targets. The viral glycoprotein mediates entry through binding to multiple attachment factors and the NPC1 receptor. Filoviruses antagonize the innate immune response through VP35, which inhibits interferon regulatory factor 3 (IRF3) activation, and VP24, which blocks interferon signaling by inhibiting STAT1 nuclear translocation. The 2014–2016 West African Ebola epidemic, the largest in history with over 28,000 cases, highlighted deficiencies in global health infrastructure and the importance of community engagement, contact tracing, and safe burial practices in outbreak control. The development of the rVSV-ZEBOV vaccine, which demonstrated 100% efficacy in a ring vaccination trial, represents a major milestone in filovirus preparedness.
Flaviviruses
Flaviviruses are positive-sense RNA viruses transmitted primarily by mosquitoes and ticks that cause significant global disease burden. Dengue virus, with four serotypes (DENV1–4), infects an estimated 390 million people annually, causing disease ranging from mild febrile illness to severe dengue hemorrhagic fever and dengue shock syndrome. Antibody-dependent enhancement (ADE), where pre-existing non-neutralizing antibodies facilitate viral entry into Fc receptor-bearing cells, complicates dengue pathogenesis and vaccine development. Zika virus, which emerged dramatically in the Americas in 2015–2016, causes mild disease in most adults but can cause congenital Zika syndrome including microcephaly in infants born to infected mothers and Guillain-Barré syndrome in adults. Yellow fever virus, despite the existence of an effective vaccine since 1937, continues to cause outbreaks in Africa and South America due to insufficient vaccination coverage. The flavivirus genome encodes three structural proteins (capsid, premembrane, envelope) and seven nonstructural proteins, with NS5 serving as the RNA-dependent RNA polymerase and NS3 functioning as a protease and helicase.
Henipaviruses
Henipaviruses, including Hendra virus (HeV) and Nipah virus (NiV), are paramyxoviruses with high pathogenicity that emerge from fruit bats (Pteropus species). Nipah virus causes encephalitis and respiratory disease with a case fatality rate of 40–75%, and outbreaks occur regularly in South and Southeast Asia, particularly Bangladesh and India, often associated with consumption of date palm sap contaminated by bat excreta. The viral G protein binds to ephrin-B2 and ephrin-B3 receptors, which are conserved across mammalian species, contributing to the broad host range of henipaviruses. Human-to-human transmission of Nipah virus occurs through close contact, particularly in healthcare settings, requiring rigorous infection control measures. No approved vaccines or therapies are currently available for henipaviruses, although experimental vaccines and monoclonal antibody therapeutics are under development.
