It wasn't chance, it was fundamental to the starting conditions of the two continents and the nature of the exchange.
First there were just more old world diseases which could infect humans than new world diseases. Prior to contact, the old world had about 80 to 90% of the world's total population, so a priori if a disease was to emerge in humans it was overwhelmingly more likely to emerge there. On top of that the old world saw much more domestication of livestock (and particularly livestock animals that had evolved alongside great apes) which in turn created more opportunities per capita for diseases to jump to humans.
For the exchange itself, Europeans were building the ships and sailing them to the New World. It was primarily Europeans on those ships, and especially those passengers who would be most likely to be incubating diseases (the old, young, and infirm) were overwhelmingly more likely to be Europeans headed west. Europeans afflicted with diseases after they reached the Americas would generally not have been allowed on ships back to Europe, or if they were they would likely not survive the voyage. Syphilis likely came from the new world, and it's easy to see how, but if there had been some American equivalent to small-pox it would simply have not been able to make the journey before going extinct as the native populations harboring it were wiped out by Old World diseases.
For it to have been "the other way around" the Americas would have needed a vastly different ecology and the people living there would have needed to beat the Old World to the level of technological development necessary to make contact.
This book came up so many times in school, always as some way to say Eurasians never had any inherent advantages. It's not that I think they do, it's just that I'm wary of people trying to reach a conclusion only because it's politically correct.
unless you plan to read a dozen academic books about a variety of topics, i dont really think there is a better option for a popular audience that will answer the question
"why were Eurasians able to so easily take over the Americas" than this book.
Within five years as many as 15 million people – an estimated 80% of the population – were wiped out in an epidemic the locals named “cocoliztli”. The word means pestilence in the Aztec Nahuatl language.
