Why was the native population so significantly affected?

The obvious reason is because of European settlement. Why did an entire civilization suddenly start diminish beginning in the 1400's? The introduction of foreign disease first by the spanish and the conquistadors. more than half of the original population died from disease that they had never encountered so their immune systems were not fully equipt to handle it. But the population continued to diminish. To explain this we must look at the public opinion of Native Americans both our recent history and in the 1600's onwards. Indians were stereotyped to be theses savage, animal like, superstitious, simple minded beings. If the majority of the new European settlers believed such things then they would have no problem hearing that a group of Indians had been brutally beaten and murdered while being driven off their land by the brave European Armies. Also due to public opinion many “expeditions” were held, the purpose, people would claim, would be to explore new lands in this foreign country, however it was more often than not that the land being explored would be where Native Americans had their homes/ villages, hunting grounds, and their own existance.