It’s because the Spanish had exposure to lots of other civilizati…
It’s because the Spanish had exposure to lots of other civilizations. Nothing breeds human diseases like other humans. The more and various human civilizations you have come into contact with, the more diseases (and antibodies) you (as a culture) develop. So…Im sure that some spanish were affected by a few rare diseases that the Peruvians had since become immune to, but most could fight them off. On the other hand, the Peruvians were exposed to hundreds of diseases that the Spaniards had since built immunities to (including some particularly nasty ones like smallpox, etc) that they couldn’t fight off.
Remember that when smallpox (or any other "big" disease) first attacks a new culture that has a hard time fighting it, it decimates them. If the population of that culture is small enough, then not enough people will survive to continue the civilization.