It... doesn't really work like that. Afro-Eurasian were not pathogenic themselves, they simply acted as vectors for bringing pathogens to the Americas. Those newly introduced diseases did what newly introduced diseases do, which is rapidly infect a population that had no prior exposure and thus no immune individuals. The introduction of disease was not, however, a singular incident. Each illness has its own, often obscure and hard to pinpoint, history of introduction to the Americas, and there could be new introductions into distinct areas and multiple re-introductions to the same area.

For a case study, let's look at the arrival of smallpox in Mesoamerica in 1520. The index case was an African servant/slave who arrived with the expedition that had been sent to arrest Cortes. Clearly, that did not work out, and most of that expedition joined up with Cortes, including this infectious man. Smallpox, however, had already made it to the Caribbean in the early 1500s and had been leap-frogging its way from island to island for almost 2 decades before finally making it to Mesoamerica.

Prior to this event, the was no smallpox in Mesoamerica, not even among the Spanish. The epidemic that followed the 1520 introduction did hit the Mesoamericans harder though, because it was a completely novel disease. No Mesoamerican had the chance to be exposed to smallpox when they were still protected by maternal antibodies, or to be exposed to a weakened virus, or to survive an out break. All of those things would grant lifelong immunity and all of them were things that could and did happen in areas of endemic smallpox, like medieval Europe.

The thing to keep in mind though, is that aside from a tangent into genetic bottlenecks and HLA-types which we won't get into here, Afro-Eurasians were no more naturally immune to epidemic disease like smallpox than Americans. We see this prominently in the British North American colonies, where smallpox would periodically be re-introduced and devastate the American-born population. Fenn's Pox Americana covers the impact of troops from Europe sparking a smallpox epidemic during the Revolutionary War quite well.

We can also see how European colonialists were laid low, just like Americans, by introduced diseases to which they had no prior exposure and thus no immunity, malaria and yellow fever in particular. It's for this reason that the Caribbean does have such prominent African ancestry. The indigenous population having been, in some cases, wiped out through disease and genocidal abuse, Africans (who can have innate genetic immunity to malaria via sickle trait) were brought into replace the work force. On this I would recommend McNeill's Mosquito Empires.

Afro-Eurasians acted as conduits to transfer disease from their respective endemic areas to previously pathogen free areas and populations. This happened on multiple occasions as epidemics would end and illnesses would be re-introduced. Often, the epidemics moved ahead of colonialists along trade routes and other modes of contact, so that outbreaks occurred well ahead of any European eyes. This hidden depopulation is in fact a key contributor to the idea of the Americas (particularly what is now the US/Canada) being seen as empty, or pristine landscapes devoid of human touch. The Europeans who moved into and colonized these depopulated areas, no matter how brutal there actions, were not themselves pathogens; they were vectors.