In the State of Kerala in southern India, Dr. Chandni Sajeevan, the head of emergency medicine at Kozhikode Government Medical College hospital, led the response to an outbreak of Nipah, a virus carried by fruit bats, in 2018. Seventeen of the 18 people infected died, including a young trainee nurse who cared for the first victims.

“It was something very frightening,” Dr. Chandni said. The hospital staff got a crash course in intensive infection control, dressing up in the “moon suits” that seemed so foreign in the pre-Covid era. Nurses were distraught over the loss of their colleague.

Three years later, in 2021, Dr. Chandni and her team were relieved when the bat breeding season passed with no infections. And then, in May, deep into India’s terrible Covid wave, a 12-year-old boy with a high fever was brought to a clinic by his parents. That clinic was full, so he was sent to the next, and then to a third, where he tested negative for Covid.

But an alert clinician noticed that the child had developed encephalitis. He sent a sample to the national virology lab. It swiftly confirmed that this was a new case of Nipah virus. By then, the child could have exposed several hundred people, including dozens of health workers.

The system Dr. Chandni and her colleagues had put in place after the 2018 outbreak kicked into gear: isolation centers, moon suits, testing anyone with a fever for Nipah as well as Covid. She held daily news briefings to quell rumors and keep the public on the lookout for people who might be ill — and away from bats and their droppings, which litter coconut groves where children play. Teams were sent out to catch bats for surveillance. Everyone who had been exposed to the sick boy was put into 21 days of quarantine.

“Everyone, ambulance drivers, elevator operators, security guards — this time, they knew about Nipah and how to behave not to spread it,” she said.