India, June 12 -- A 43-year-old man from Kerala's Kozhikode district is critically ill on ventilator support with a suspected Nipah infection, again raising questions over how the deadly virus repeatedly spills over from bats to humans in the state.

Health officials have started contact tracing and containment measures without waiting for final laboratory confirmation from the National Institute of Virology in Pune. Five primary contacts have been identified and placed under observation, officials said.

For Kerala, the response has become familiar. Since its first Nipah outbreak in 2018, the state has built one of India's strongest systems for detecting, isolating and containing the infection.

But the virus itself remains poorly unders...