A mysterious disease outbreak is slowly killing the members of a South American tribe, who came in contact with the outside world in the mid 20th century.

The epidemic may wipe out the Ayoreo-Totobiegosode tribe from Paraguay's Chaco region if remained untreated, tribal rights group Survival International has warned on World Health Day.

Totobiegosode are the most isolated sub groups of Ayoreo tribe of the Paraguayn Chaco, who were first contacted in 2004.

Totobiegosode people live in small communities and grow squashes, beans and melons in the sandy soil, and hunt in the forest for a living.

The members of the tribe have a rare respiratory disease that shows symptoms of TB-like illness.

"The deadly epidemic threatens to wipe out Paraguay's recently contacted Ayoreo tribe, and sets a deadly precedent for their relatives still hiding in the forests, who are the last uncontacted Indians outside the Amazon," the organisation said in a statement.

The Ayoreo people are threatened by cattle ranchers who burn the Indians' land to establish their ranches. Several tribal people have been forced out of their forests in the process since 1969, some as recently as 2004.

Almost all the Ayoreo members, who have been forced out of their lands, have suffered from the rare disease.

"The most recent Ayoreo victim killed by the respiratory disease, Chiri Etacore, was forced out of the forest in 1986. Chiri died in October 2013," Survival International said.

An unknown number of relatives of these indigenous people are still uncontacted and are threatened by the epidemic.

"When uncontacted people are forced into contact with outside society disease swiftly follows. Here is proof that forced contact is nothing more than a death sentence for tribal peoples," Survival's director Stephen Corry said.

A 2013 report by the University of Maryland revealed that the Paraguayan Chaco has the fastest rate of deforestation in the world and the land inhabited by the Ayoreo is some of the last standing forest in the region.

However, most of the ancestral lands of these indigenous people have been brought by Brazilian cattle-ranching businesses that are illegally encroaching into Ayoreo's territory, putting the tribe at the brink of being forced out of forests and thereby at the risk of contracting the mysterious disease.

"The government is doing nothing to protect the lives of these Ayoreo's uncontacted relatives. Instead it is allowing the wholesale destruction of the Chaco forest at the hands of Brazilian ranchers," Corry added.