Actually, the Afro-Eurasian disease environment had evolved over thousands of years, with the epidemic diseases mostly evolving from diseases of domestic animals.
These diseases appeared at intervals of centuries, wiping out significant percentages of the population each time. Most of these epidemics are not recorded.
The relevant point is that the Spanish (and everybody else in the Old World) was descended from the survivors of those epidemics, and had acquired considerable immunity as a result.
When the disease ecologies of the two hemispheres merged, the native Americans were within a decade or two exposed to all the pathogens people of the old world had 10,000 years to adapt to. IOW, the Spanish were more highly evolved than the Indians, in their resistance to disease.
It is probable, though debated, that syphilis evolved in America and made the voyage the other way.
My basic point was that people from around the world have been subject to diseases that have wiped out large shares of various populations. It was not restricted to the new world. The leftists who love to condemn Europeans for every ill in the world refuse to recognize that millions of Euros have been killed either by disease or invaders from the east and south.