It is estimated, based on archaeological data and written records from European settlers, that from 8 to 140 million indigenous people lived in the Americas when the 1492 voyage of Christopher Columbus began a historical period of large-scale European interaction with the Americas. European contact with what they called the "New World" led to the European colonization of the Americas, with millions of emigrants (willing and unwilling) from the "Old World" eventually resettling in the Americas.

While the population of Old World peoples in the Americas steadily grew in the centuries after Columbus, the population of the American indigenous peoples plummeted. This was somewhat caused by direct conflict and warfare with European colonizers and other Native American tribes, but probably mostly due to their susceptibility to old world diseases [smallpox, influenza, bubonic and pneumonic plagues, etc.] that they had never before been exposed to. The extent (and to a lesser extent the causes) of this population decline have long been the subject of debate.

For more information about Population history of American indigenous peoples, read the full article at Wikipedia.
This text uses material from Wikipedia and is available under the GNU Free Documentation License.