Smallpox, measles, bubonic plague, diphtheria, typhus -- these were the silent weapons that caused such a remarkable decline in the population. The locals had not had contact with these diseases until then, so they had not developed immunity to them. But why did the same not happen with the European conquerors? Didn't they find any new deadly disease for them?

You have to go back and look for the answer, to the Neolithic. During the Neolithic Revolution, collectors and hunters became producers, dedicating themselves to agriculture and livestock. Nomadic groups were sedentary and the population increased. Stable high-density groups were herds for viruses and germs. By their choice, they did not have to resist a single guest and developed the possibility of going for others; infectious diseases were much more contagious and virulent in the new society.

Of course, all of this also happened in America, but in Eurasia zoonoses had much more effect than on the other side of the ocean. They domesticated large mammals, adding animal density to high human density. Thus, the most deadly contagious diseases for humans evolved from animal diseases: smallpox, influenza, tuberculosis, malaria, plague, measles and cholera, which evolved mainly from cattle, pigs and sheep diseases.

In America, during the last glaciation of 13,000 years ago, many species of large mammals were lost and, subsequently, humans would only tame four species of animals: turkey, llama, akuria and duck; and in much smaller quantities than in Eurasia. That is why, when the European conquerors arrived, violent Eurasian diseases were unleashed that did not await them.

What would have happened if contagious diseases had also been found in America? Would this paralyze conquest and colonization? If you look at the example of Africa, no. In tropical Africa there were also such diseases, mainly yellow fever, which significantly affected European colonizers. This resulted, at the most, in a slowdown in the process, which, as we know, did not paralyse.