In mid-August it was reported that the Mashco-Piro, an indigenous tribe in the Peruvian Amazon, has been trying to make contact with outsiders, possibly over anger at logging encroaching on their territory. In the past, the Mashco-Piro have resisted interaction with strangers, avoiding and sometimes killing any they encounter. The news raised questions about how such tribes still exist and how Western societies should respond to them.

• How many uncontacted tribes are still left?

No one knows for sure. At a rough guess, there are probably more than 100 around the world, mostly in Amazonia and New Guinea, says Rebecca Spooner of Survival International, a London-based organization that advocates for the rights of indigenous peoples. Brazil’s count is likely to be the most accurate. The government there has identified 77 uncontacted tribes through aerial surveys and by talking to more Westernized indigenous groups about their neighbors.

There are thought to be around 15 uncontacted tribes in Peru, a handful in other Amazonian countries, a few dozen in the Indonesian part of the island of New Guinea and two tribes in the Andaman Islands off the coast of India. There may also be some in Malaysia and central Africa.

• Have they really had no contact with the outside world?

Most have had a little, at least indirectly. “There’s always some contact with other isolated tribes, which have contact with other indigenous people, which in turn have contact with the outside world,” says Spooner.

Many of the Amazon tribes choose to avoid contact with outsiders because they have had unpleasant encounters in the past. The Mashco-Piro, for example, abandoned their settled gardens and fled into the forest. According to Glenn Shepard, an ethnologist at the Emilio Goeldi Museum in Belem, Brazil, this happened after rubber companies massacred tribespeople at the turn of the 20th century.

Some researchers refer to such tribes as “voluntarily isolated,” rather than uncontacted.

• Why would tribes choose to end their isolation?

Often, they feel forced out by encroaching civilization, says Spooner. Survival International has documented cases where settlements have been bulldozed and tribespeople harassed or killed. This leaves the survivors feeling like they have no option but to give up.

Others suggest that tribes may seek contact with outsiders because they begin to trust their intentions, Hill says.

• What happens then?

Often, there is a lot of disease because the tribespeople are exposed to novel pathogens. It is not uncommon for half the population to die of respiratory illness, unless outsiders bring sustained medical care, Hill says.