Abstract
All animal life on earth is thought to have a common origin and have common genetic mechanisms. Evolution has enabled differentiation of species. Pathogens likewise have evolved within various species and mostly come to a settled dynamic equilibrium such that co-existence results (pathogens ideally should not kill their hosts). Problems arise when pathogens jump species because the new host had not developed any resistance. These infections from related species are known as zoonoses. COVID-19 is the latest example of a virus entering another species but HIV (and various strains of influenza) were previous examples.