How Did the Global Transfer of Disease in Pre-Modern World Helped in Colonisation of the Americas?

The Portuguese and the Spanish conquered America not just with the conventional military weapons but also with the germs such as the small pox.

The small pox was carried on their person (Spanish invaders were immune to this disease) to America for which the American Indians were not immune to and proved a deadly weapon as it killed the whole community there.

These diseases could not be bought or captured and turned against the European invaders unlike the guns and other conventional weapons.