India has curbed public gatherings and shut some schools in the southern state of Kerala after two people died of Nipah, a virus from bats or pigs that causes deadly fever, officials said Thursday.

The virus has no vaccine and a fatality rate ranging from 40 to 75 percent, according to the World Health Organization.

Symptoms include intense fever, vomiting and a respiratory infection, but severe cases can involve seizures and encephalitis, an inflammation of the brain, and result in a coma.

Three others have tested positive, and more than 700 people including 153 health workers who came in contact with those infected are under observation, health officials said.

At least four people have been hospitalized, including the nine-year-old child of one of the victims.

Initially transmitted from animals such as fruit bats or pigs, Nipah is also spread from person to person, the WHO said.

The incubation period — the time from infection to the onset of symptoms — ranges from around four to 14 days, but has been reported to be as long as 45 days, according to the WHO.

In 2018, at least 17 people died after being infected by the virus in Kerala.

The virus was first identified in 1998 after it spread among pig farmers in Malaysia. In India, the first Nipah outbreak was reported in the state of West Bengal in 2001.

The WHO lists Nipah as one of its priority diseases that pose "the greatest public health risk due to their epidemic potential" and where there is "no or insufficient countermeasures".