system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: if European diseases killed 90% of indigenous N & S Americans, why didn't American diseases return the favor when European ships returned?
gpt
Europeans had built immunity to a wider array of pathogens over the centuries, from Black Plague, etc., etc., because their range of territory covered was so much wider.

By 1500, Europeans had been attacked by Jihadists from North Africa coming by boat across the Mediterranean, had traveled to the Holy Land to return the favor via the Crusades, had trade voyages of Marco Polo, had experienced invasions by the Mongols and the Huns, etc.  Not to mention 1500 years of pan-European contact.

Whereas the typical NA Indian had primarily traded only with adjacent tribes, and there was no widespread large-ship-building and seafaring tradition.

The Europeans were inoculated to a variety of diseases; the NA Indians, to practically none.