From Guns, Germs, and Steel, explain why the exchange of diseases from the old world and the new world was so one-sided?Europe is the old world and...

The exchange of diseases was so one-sided because the
people of the Americas did not have many large, domesticated animals while the people of
the Old World had such things as pigs and horses and
cows.

This matters because of the fact that the diseases
that killed the natives of the Americas were generally ones like smallpox that had come
from the domesticated animals that were kept by people in the Old World.  The Old World
had been luckier so they had domesticated animals.  The domesticated animals gave them
the germs that killed off the people of the New World.