With tests from other suspect deaths awaited, authorities in Kerala state have ordered emergency measures to control the virus, which is spread by fruit bats.
Three of the fatalities are members of the same family -- dead bats were found in a well at their home. A nurse who treated one of the family has also died, leaving a heart-wrenching note for her family.
"We sent 18 samples for testing. Out of these 12 tested positive. Ten of those who tested positive have died and the remaining two are undergoing treatment," a health official in Kerala's Kozhikode district, the centre of the outbreak, told AFP.
Ninety-four people who have come into contact with those who died have been isolated in their homes.
"They have been quarantined as a precaution," Kerala state health surveillance officer K.J. Reena told AFP.
Nipah has killed more than 260 people in Malaysia, Bangladesh and India since 1998 and has a mortality rate of nearly 70 percent, according to the World Health Organisation.
There is no vaccination for the virus which induces flu-like symptoms that lead to an agonising encephalitis and coma.
The WHO has named Nipah as one of the eight priority diseases that could cause a global epidemic, alongside the likes of Ebola and Zika.
