it is often stated that a great percentage of Indians died of the diseases brought by the Spanish as they weren't immune to these. Q.: Why didn't the same thing happen to the Spanish vice versa?
Regarding "New World" diseases When Europeans brought diseases to the new world, how come Europeans themselves didn't get sick from diseases specific to the new world? written by u/anthropology_nerd
Not exactly what you asked, but regarding the extent of Old World diseases in the New World: Disease in 'The New World' with answers by u/Reedstilt and u/anthropology_nerd