Before Europeans came to the hemisphere there were about 80% more indigenous people.
Small pox is a bitch.
By the time we settled the US, it was practically empty.
It was not Columbus’ fault, per se. As Europeans travelled around the globe infections such as this were inevitable.
Mexico suffered a severe population decline a few decades after the conquest but I read something to the effect that it was caused largely by diseases that were already present before the Spanish arrived.