The power of the image. From National Geographic (via BB):

“We are very confident the photos are genuine,” said Miriam Ross, a spokesperson for Survival International, which estimates that half of the hundred or so uncontacted tribes in the world live in the rain forests of Brazil and Peru.

Some experts say few, if any, tribes have had no outside contact. It’s more likely is that previous generations had negative encounters, prompting social taboos that continue to drive clans deeper into isolation.

Due to their vulnerable immune systems, these groups are highly susceptible to diseases borne by outsiders such as missionaries, loggers, or oil workers.