A hostile disease environment greeted Europeans on their arrival in Africa hampering their efforts at colonization there for several hundred years; but the relatively benign set of infections encountered by the Spanish in the New World did little to hinder their invasion. In fact, the absence of any new, virulent diseases (with the possible exception of syphilis) allowed Europeans to conduct their exploratory and military expeditions free from the threat of biological reprisals. At the same time, the diseases that the Spanish brought with them from the Old World, more than any other factor, aided them in their conquest of Amerindian societies. The inadvertent introduction of smallpox, measles, and influenza to nonimmune populations claimed millions of lives and in so doing weakened the resistance of indigenous society, leaving it vulnerable to penetration by Europeans. Once the period of military conquest had ended, however, epidemic disease worked against Spanish interests by significantly reducing the number of Indians available for tribute and labor. Their most valuable ally had become an enemy, and throughout the remainder of the sixteenth century, disease continued to reduce the profits of empire.

The Spanish conquest and the introduction of previously unknown diseases unleashed a series of disasters on Indian Ecuador that weakened the foundations of native communities but ultimately proved incapable of destroying them. The demographic effects of sixteenth-century pandemics are undoubtedly the most obvious, but analysis of these events also reveals social and economic consequences of disease and population loss.