The number of pathogens known to infect humans is ever increasing. Whether such increase reflects improved surveillance and detection or actual emergence of novel pathogens is unclear. Nonetheless, infectious diseases are the second leading cause of human mortality and disability-adjusted life years lost worldwide. On average, three to four new pathogen species are detected in the human population every year. Most of these emerging pathogens originate from nonhuman animal species.
Zoonotic pathogens represent approximately 60% of all known pathogens able to infect humans. Their occurrence in humans relies on the human-animal interface, defined as the continuum of contacts between humans and animals, their environments, or their products. The human-animal interface has existed since the first footsteps of the human species and its hominin ancestors 6–7 million years ago, promoting the prehistoric emergence of now well-established human pathogens. These presumably include pathogens with roles in the origin of chronic diseases, such as human T-lymphotropic viruses and Helicobacter pylori, as well as pathogens causing major crowd diseases, such as the smallpox and measles viruses and Bordetella pertussis. Since prehistory, the human-animal interface has continued to evolve and expand, ever allowing new pathogens to access the human host and cross species barriers.