By a strict definition, zoonotic pathogens (accounting for about 60 percent of our infectious diseases, as I’ve mentioned) are those that presently and repeatedly pass between humans and other animals, whereas the other group of infections (40 percent, including smallpox, measles, and polio) are caused by pathogens descended from forms that must have made the leap to human ancestors sometime in the past. It might be going too far to say that all our diseases are ultimately zoonotic, but zoonoses do stand as evidence of the infernal, aboriginal connectedness between us and other kinds of host.
