I was thinking about this because of the following reasons:
American epidemics: After the contact, majority of native population of Americas died from the epidemics of smallpox, etc. However the affects on Old World population were lighter.
Origins of Black Death: As far as I know, there are still many unanswered questions about the pathological origins of the plague except its connection with Yersinia pestis and that it started to spread possibly somewhere in China.
Polynesian contact: It is widely accepted that there has been a contact between polynesian and pre-colombian american cultures.
So my "educated guess" is that the Polynesian sailors could have been brought the disease with them from Incan people who are immune to it to the Chinese ports (or Southeast Asian ports and then Chinese ones) and that way started the wave of epidemic that is comparable to that of native american one in size. This could also explain the relatively smaller impact of epidemics on Europens after Colombian contact.
I am not a historian and have no expertise on the subject. I am asking you to please show me why this theory can or cannot work?
Except this is the opposite of what happened for a couple of reasons.
The key problem with this proposal is that Yersinia pestis has also been identified as the cause of the Plague of Justinian in the 6th century.
The other big issue is that the bubonic plague was also brought to the New World in the Columbian Exchange. It didn't exist in the New World prior to that.
There's a difference between immunity and minimal impact. The Old World brought smallpox with them, but smallpox still manifested into disease in the Old World. Bubonic plague was unattested in the New World prior to the Columbian Exchange.
In terms of epidemics brought by contact, don't forget about the Syphilis outbreak of 1494, which was also a result of the Columbian Exchange. And genomic sequencing demonstrated that the bacterium in question, Treponema pallidum, has its origins in the mountains of South America and gradually worked its way to the coast, where there was contact with Old Worlders.
But yeah, there's no evidence of Yersinia pestis in the New World prior to that, as well as good evidence that it was in the Old World centuries prior to contact.