By Subir Bhaumik
BBC correspondent, Calcutta
Nearly 50 children and several adults from an isolated tribe in India's Andaman and Nicobar Islands have contracted measles in the past month.
The Jarawas - one of a number of at-risk tribes people (Photo: Pankaj Seksaria)
Doctors say that unless controlled, the illness could seriously affect the Jarawa tribe which once numbered 5,000 and is now down to about 270 people.
They say more than 20% of the tribe has been affected by the epidemic.
Samir Acharya, convenor of the Society for Andaman and Nicobar Ecology (Sane) has warned the matter is now "serious".
Mr Acharya confirmed that 47 Jarawa children were affected by the epidemic.
Initially officials said that the Jarawas were only affected by "heat rash".
But now doctors say many have been admitted to the Pant Government Hospital in Port Blair, capital of the Andaman and Nicobar Islands.
They have pointed out that while no deaths have so far occurred, measles has over the past two centuries decimated tribes on the islands.
In the 19th century, the disease killed at least half of the Great Andamanese on one island and all those on another, said Survival International, which campaigns for tribal groups.
"That tribe, also once 5,000-strong, now numbers only 41 people."
When 108 Jarawas contracted measles in 1999, the authorities first denied the existence of the outbreak, but later were forced to concede the illness existed after testimony from doctors on the islands, Survival International said.
Environmentalists say that exposure to outside influences, including disease, is putting the Jarawas at risk.
The local authorities in the Andamans have been criticised by environmentalists and non governmental organisations for not taking enough steps to protect the tribes and preserve their habitat.