How did Pizarro and his small army of mercenaries, totaling less than 400, conquer what was becoming the world's largest civilization? Much of the "conquest" was accomplished without battles or warfare as the initial contact Europeans made in the New World resulted in rampant disease. Old World infectious disease left its devastating mark on New World Indian cultures. In particular, smallpox spread quickly through Panama, eradicating entire populations. Once the disease crossed into the Andes its southward spread caused the single most devastating loss of life in the Americas. Lacking immunity, the New World peoples, including the Inca, were reduced by two-thirds.