It should also be noted that human populations are incredibly resilient to epidemics. In the absence of "other things", populations suffering catastrophic virgin soil epidemics will typically rebound to pre-epidemic levels in decades. It's not a sufficient explanation for the centuries-long decline of indigenous American populations. The black death was no less severe and successor epidemics continued throughout Europe in the 15th century, yet we see nothing like the demographic collapse of the Americas post-contact.