There are a number of tribes along the Amazon who have never been contacted by the outside World, for example Brazil actively tries to identify isolated Indian tribes by fly-over but a policy to not contact them. Here is a story about drug traffickers massacring one of the documented, but uncontacted tribes: http://www.ibtimes.com/uncontacted-amazon-tribe-massacred-pe...

Just to clarify: in Brazil, those "uncontacted" tribes include many (probably a majority) that have had contact with the outside world in the past. However, they currently have no ongoing contact, and the Brazilian authorities don't have enough data about them from the previous attempt at contact.

In some cases, they might have had a less than pleasant experience when they were first contacted by explorers or loggers. Nowadays, while they'd be in danger should they meet illegal loggers and the like, it shouldn't be dangerous for them to contact the authorities. However, their memory from the initial encounter might have been passed on for generations.

Also, as you say, the Brazilian agency in charge of documenting and protecting the Indians has a policy of not forcing contact. So those tribes can remain uncontacted indefinitely.

The page below has more info. Use your favorite translator.

http://mundoestranho.abril.com.br/materia/ainda-existem-trib...

Many of these Brazilian tribes have had some contact with loggers/miners/etc, just no formal government contact.

Perhaps more interesting is the case of the Sentinelese, who live on islands in the bay of bengal, and may have had literally no spoken contact with outsiders in hundreds of years:

https://en.wikipedia.org/wiki/Sentinelese_people

There have been many visits on the shore of their island by Indian anthropologists.