Native American Populations Contracted Many Diseases from the Invading Europeans

Native American populations contracted many diseases from the invading Europeans. Why did the Europeans not contract diseases from the Native Americans?

A) The vertical landmass of America is much less adapted to support epidemic disease than the long horizontally stretching continent of Eurasia.
B) The European farmers' interactions with livestock created ideal situations for zoonosis while the Native Americans' interactions with livestock did not.
C) The explorers who came over had already been exposed to, and so had already gained immunity to, American diseases.
D) European immune systems were more highly developed than Native Americans'.
E) All of the answers are correct.