Encounters between European explorers and the Amerindians exposed the latter to a variety of diseases of extraordinary virulence. Having migrated from Northeastern Asia 15,000 years ago, the Amerindians were hitherto not introduced to the plethora of contagious diseases that emerged after the rise of agriculture in the Old World. As such, they had immune systems that were ill-equipped to handle the diseases that their counterparts in Eurasia had grown resistant to. When the Europeans arrived in the Americas, in short order, the indigenous populations of the Americas found themselves facing smallpox, measles, whooping cough, and the bubonic plague, among others. In tropical areas, malaria, yellow fever, dengue fever, river blindness, and others appeared. Most of these tropical diseases were traced to Africa. Smallpox ravaged Mexico in the 1520s, killing 150,000 in Tenochtitlán alone, including the emperor, and Peru in the 1530s, aiding the European conquerors. A combination of Spanish military attacks and evolutionarily novel diseases finished off the Aztec Empire in the sixteenth century. It is commonly believed that the death of as much as 90% or 95% of the Native American population of the New World was caused by Old World diseases, though new research suggests tuberculosis from seals and sea lions played a significant part.