The south Indian state of Kerala declared a health alert on Monday after at least three people died from the deadly Nipah virus.
Eight other deaths in Kerala are being investigated for possible links to the virus, which has a 70 per cent mortality rate and is carried mainly by fruit bats.
State Chief Minister Pinarayi Vijayan has urged citizens to stay vigilant and follow instructions from the health department.
"Health department is doing everything possible to save the lives of the infected & prevent the advance of virus," his office said on Twitter.
The victims all died in Calicut district, said state health secretary Rajeev Sadanandan.
Samples tested in government labs confirmed the presence of the Nipah virus in three deaths, and blood and body fluid samples from the suspected cases had been sent for testing.
"It will take 24-48 hours for the results to come," Mr Sadanandan said.
India's health minister sent medical experts to Kerala after a local politician reported that residents of Calicut were panicking.
The team would "initiate required steps as warranted by the protocol for the disease", J P Nadda said on Twitter.
The Press Trust of India news agency said two of the confirmed victims were siblings in their early 20s.
A nursing assistant who had treated them died on Monday and the father of the victims was being treated in hospital, PTI reported.
Neighbours told local media the siblings had eaten fruit picked from a compound where they were building a home.
A bat was found in the well of their home which was later sealed, PTI quoted state health minister K K Shylaja as saying.
Nipah induces flu-like symptoms that often lead to encephalitis and coma. Fruit bats are considered the main carrier of the virus, for which there is no vaccination, according to the World Health Organisation.
The virus was first identified in Malaysia in 1998. It spread to Singapore and more than 100 people were killed in both countries. Pigs were the virus hosts on that occasion, but they are believed to have caught it from bats.
In India the disease was first reported in 2001 and again six years later, with the two outbreaks claiming 50 lives.
Both times the disease was reported in areas of the eastern state of West Bengal bordering Bangladesh.
Bangladesh has borne the brunt of the disease in recent years, with more than 100 people dying of Nipah since the first outbreak was reported there in 2001.
In 2004, people in Bangladesh became infected with Nipah after eating date palm sap that had been contaminated by fruit bats.