Most new pandemics have originated through the “zoonotic” transmission of pathogens from animals to humans (Murphy 1998; Woolhouse and Gowtage-Sequeria 2005), and the next pandemic is likely to be a zoonosis as well.
Zoonoses enter into human populations from both domesticated animals (such as farmed swine or poultry) and wildlife. But many historically significant zoonoses were introduced through increased human-animal interaction following domestication, and potentially high-risk zoonoses (including avian influenzas) continue to emerge from livestock production systems (Van Boeckel and others 2012; Wolfe, Dunavan, and Diamond 2007).
Some pathogens (including Ebola) have emerged from wildlife reservoirs and entered into human populations through the hunting and consumption of wild species (such as bushmeat), the wild animal trade, and other contact with wildlife (Pike and others 2010; Wolfe, Dunavan, and Diamond 2007).
Zoonotic pathogens vary in the extent to which they can survive within and spread between human hosts. As shown in table 17.2, the degree of zoonotic adaptation spans a continuum from transmission only within animal populations (stage 1) to transmission only within human populations (stage 5).
Most zoonotic pathogens are not well adapted to humans (stages 2–3), emerge sporadically through spillover events, and may lead to localized outbreaks, called stuttering chains (Pike and others 2010; Wolfe and others 2005). These episodes of “viral chatter” increase pandemic risk by providing opportunities for viruses to become better adapted to spreading within a human population.
Pathogens that are past stage 3 are of the greatest concern, because they are sufficiently adapted to humans to cause long transmission chains between humans (directly or indirectly through vectors), and their geographic spread is not constrained by the habitat range of an animal reservoir.