Portugal established a route to China in the early 16th century, sending ships via the southern coast of Africa and founding numerous coastal enclaves along the route. Following the 'discovery' of the New World in 1492 by Christopher Columbus and the first circumnavigation of the world by Juan Sebastián Elcano and Ferdinand Magellan in 1521 (a voyage promoted by the Spanish Crown of Castile), expeditions led by conquistadors in the 16th century established trading routes linking Europe with all these areas.
Human infections gained worldwide transmission vectors for the first time: from Africa and Eurasia to the Americas and vice versa. The spread of old-world diseases, including smallpox, flu and typhus, decimated the inhabitants of the New World.
The conquistadors were professional warriors, using European tactics, firearms, and cavalry. Their units (compañia, companhia) would often specialize in forms of combat that required long periods of training that were too costly for informal groups. Their armies were mostly composed of Iberian and other European soldiers.
Native allied troops were largely infantry equipped with armament and armour that varied geographically. Some groups consisted of young men without military experience, Catholic clergy which helped with administrative duties, and soldiers with military training. These native forces often included African slaves and Native Americans. They not only fought in the battlefield but served as interpreters, informants, servants, teachers, physicians, and scribes. India Catalina and Malintzin were Native American women slaves who worked for the Spaniards.