The zoonotic origins of pandemics

The disease emergence model above provides a construct for how pathogens emerge from animals and illustrates the continuum of animal pathogen infectivity in the human population. However, relatively little is known about the factors that mediate transition from one stage to the next as a pathogen of animal origin scales the stages of this paradigm Figure1, ever increasing its ability to reside in the human population and be transmitted throughout it. What is known, however, is that the interface between humans and animals is of paramount importance in the process. As we increase our interactions with animals through hunting, the trading of animal foods, animal husbandry practices, wet markets, and the domestication of animals or exotic pets, the probability of cross-species transmission dramatically increases.

Zoonotic disease emergence model outlining the 5 stages of pathogen emergence from animals to humans.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2874076/