The America that greeted the first Europeans was, thus, far from an empty wilderness. It is now thought that as many people lived in the Western Hemisphere as in Western Europe at that time -- about 40 million. Estimates of the number of Native Americans living in what is now the United States at the onset of European colonization range from two to 18 million, with most historians tending toward the lower figure. What is certain is the devastating effect that European disease had on the indigenous population practically from the time of initial contact. Smallpox, in particular, ravaged whole communities and is thought to have been a much more direct cause of the precipitous decline in the Indian population in the 1600s than the numerous wars and skirmishes with European settlers.