By the mid-sixteenth century, the Portuguese and Spanish colonization of America began in a decisive way. But the conquest could not be facilitated because of arms and ammunition but because of a disease. Europeans had been exposed to small pox and hence they had developed immunity against this disease. But the Americans had been isolated from the world and they had no immunity against small pox. When the Europeans reached there, they carried the germs of small pox alongwith them. The disease wiped off the whole communities in certain parts of America. And thus, the Europeans could easily get control of the Americas.