This is one of the most incredible stories I've read recently. Apparently tribes still exist in the rainforest of Peru and Brazil that (in the year 2008!) have never been contacted by the outside world. Here is a picture taken by a plane flying over the area, which they must have thought to be a bird or evil spirit.
From the article,
"It is extraordinary to think that, in 2008, there remain about a hundred groups of people, scattered over the Earth, who know nothing of our world and we nothing of theirs, save a handful of brief encounters."