The health risk associated with pathogens emergence depends on a combination of socioeconomic, environmental, and ecological factors that affect the virulence or the pathogenic potential of microbes and their ability to infect susceptible host populations.
The emergence and transmission of an infectious disease typically follows a pathway that involves a reservoir (i.e., the environment, a vector, or a secondary host) to contact a native host (i.e., human or animal species). The risk of disease emergence thus greatly depends on the rate of contact and/or spillover between reservoirs and the native host (Lambin, Tran, Vanwambeke, Linard, & Soti, 2010; Nii‐Trebi, 2017).
In zoonoses, which represent the majority of emerging infectious diseases, contact between wildlife reservoir species and livestock or humans creates interfaces that might be important for the transmission of pathogens.
Many synanthropic species (i.e., species, animals, or plants that live near humans), such as rodents, birds, bats, and certain other mammal species, have been shown to carry zoonotic pathogens and in some cases act as reservoir hosts for these pathogens.
Anthropogenic pressures associated with urbanization often bring these species into closer contact with livestock and humans and thus favor disease emergence (Hassell, Begon, Ward, & Fevre, 2017). Another important factor influencing human–livestock–wildlife contact is livestock husbandry. Informal livestock raising is commonplace in African cities and is often characterized by low biosecurity and mixed‐species livestock being kept in close proximity to humans.
Evidence from recent zoonotic emergence events in Asia (such as avian influenza viruses; World Health Organization Global Influenza Program Surveillance Network, 2005) and the circulation of relatively stable zoonoses (such as bovine tuberculosis; Gortazar et al., 2011) implicate a role for livestock acting as bridge hosts, epidemiologically linking wildlife and humans.
Sociodemographic factors, such as population density, migration, trade, conflicts, social instability, or access to clean water, also have important impacts on the transmission of pathogens and vector dynamics and largely influence the epidemiology of infectious diseases (Hassell et al., 2017; Nii‐Trebi, 2017).
The chain of infection further depends on the vulnerability of host populations. Not all conspecifics are competent hosts for a given pathogen (Bento et al., 2017), and “dead‐end” hosts can play a role in regulating infection. Important factors influencing host susceptibility to infection are age and gender, but also immunocompetence and immunological history (Hassell et al., 2017).
If ancient pathogens, such as the 1918 influenza virus, were to re‐emerge, it may not have the same consequences as the past pandemics. Indeed, in the case of the 1918 influenza virus, for example, most people now have some immunity to the 1918 virus because subsequent human flu viruses are in part derived from it. And, in mice, regular flu vaccines and drugs are at least partly effective against an infection with reconstructed viruses that contain some of the genes from the 1918 flu (von Bubnoff, 2005).