When Europeans discovered Americas they also imported plagues. These plagues were one factor of the collapse of the pre-columbian cultures.
The deaths of somewhere between 40 and 100 million people during a relatively short span of time was not caused by just one disease, but several. Many references assume that the near extinction of the population was caused by European diseases for which the people of the Americas had no immunity. [...] In several regions, these diseases completely annihilated entire ethnic groups[...]
In late 1520, as the Hernan Cortes expedition waited to strike the capital of the Aztecs, Tenochtitlan, a fatal blow, small pox swept through the Aztec Empire. It is estimated that 40% of its population died of the disease. [...]Without the devastation of the smallpox epidemic, it is far less likely that the Spanish and their native allies would have been able to conquer the Aztec Empire.
By 1528, the Central American smallpox epidemic had reached the Inca Empire in South America. [...] Smallpox soon killed many of the Inca leaders and soldiers. When they saw the power of the imperial government weakened, vassal peoples rebelled. They gave the assistance to Pizzaro’s puny army that was needed to topple the Incas
The Aztecs were struck by a smallpox plague starting in September 1520, which lasted seventy days. Many were killed, including their new leader, the Emperor Cuitlahuac
The Old World diseases brought with the Spanish and against which the indigenous New World peoples had no resistance were a deciding factor in the conquest; they decimated populations before battles were even fought. It is estimated that 90% of the indigenous population had been eliminated by disease within the first century of European contact.
It were not the plagues alone, but the casualties made the empires weaker and made it possible, that European conquistadors won the day.