This is commonly repeated, but if you go look at actual record keeping - at the history of early North American and Caribbean/Bahamas colonization through the period where there stopped being as much cross-cultural contact - we see epidemic after epidemic wipe our colonials as much as any. The mortality rate is staggering - even places that should have shown this the most - Haiti for example, the documented evidence was far closer to 100% mortality to Europeans, and significantly less for indigenous. Coming to Haiti during the Haitian revolution was a death sentence for all of the people who lived in squalor in one of the densest cities in Europe - Paris.

Cultures coming together historically were deadly. There is a school of thought that argues that the Persian and Roman downfall was directly associated with the ability of diseases to transit roads.

Of course, this is the stuff that we have records for. It's possible that there was a undocumented mass extinction event, But I tend to be skeptical of historical claims that are based on extremely localized and limited archeological projections.