Diamond in "Guns, Germs and Steel" suggests that because the old world civilizations had greater population densities it allowed for more diseases to arise and resistance to them to built up.
With lower densities and widely spaced villages a disease might kill off the whole village and not get a chance to spread.
This way serious diseases can build up and along with them some degree of population immunity or at least resistance.
If you mix those 2 different populations one suffers a lot more.