Indian authorities in the northern state of Punjab have quarantined around 40,000 residents from 20 villages following a Covid-19 outbreak linked to just one man.
The 70-year-old died of coronavirus - a fact found out only after his death.
The man, a preacher, had ignored advice to self quarantine after returning from a trip to Italy and Germany, officials told BBC Punjabi's Arvind Chhabra.
India has 640 confirmed cases of the virus, of which 30 are in Punjab.
However, experts worry that the real number of positive cases could be far higher. India has one of the lowest testing rates in the world, although efforts are under way to ramp up capacity.
There are fears that an outbreak in the country of 1.3 billion people could result in a catastrophe.
The man, identified as Baldev Singh, had visited a large gathering to celebrate the Sikh festival of Hola Mohalla shortly before he died.
The six-day festival attracts around 10,000 people every day.
A week after his death, 19 of his relatives have tested positive.
"So far, we have been able to trace 550 people who came into direct contact with him and the number is growing. We have sealed 15 villages around the area he stayed," a senior official told the BBC.
Another five villages in an adjoining district have also been sealed.
This is not the first time that exposure has resulted in mass quarantining in India.
In Bhilwara, a textile city in the northern state of Rajasthan, there are fears that a group of doctors who were infected by a patient could have spread the disease to hundreds of people.
Seven thousand people in villages neighbouring the city are under home quarantine.
India has also declared a 21-day lockdown, although people are free to go out to buy essential items like food and medicine.