(Having watched the video): No, GP does mean "old-world cities with immigrants". However, GP meant to end with "that never developed in the new world". The sto

(Having watched the video): No, GP does mean "old-world cities with immigrants". However, GP meant to end with "that never developed in the new world". The story, in brief: old-world cities, which were crowded, always had a fresh supply of (non-immune) immigrants, and had lots of human/animal contact, allowed the development of particularly lethal diseases. Such diseases did not develop in the new world, which lacked such cities. Thus there was no "americapox" to spread from the new world to the old. I repeat the story without knowing/endorsing accuracy.