By Vibhuti Agarwal, Krishna Pokharel and Rajesh Roy

The village of Sarna Toli in eastern India hasn't recorded one coronavirus case. Yet the rural enclave of about 250 people has been transformed by the global pandemic.

Dozens of men who would normally be working in faraway big cities, sending cash home to their families, while away time playing cards and chatting in the village square or under trees near the local Hindu temple.

There is little work for them in the village. The loss of the money they were shipping home is a huge blow to the local economy. Worries are growing that they could introduce the coronavirus from India's cities, where it is raging, to a hinterlands area where the state-run hospital shut down last month and the sole private health-care provider is too expensive for all but the richest residents.

"Their return has created fear," said Shankar Mahli, the elected head of the village in the state of Jharkhand.

India's major cities have been hardest hit by the coronavirus, as outbreaks in Mumbai and Delhi overwhelm hospitals stretched in the best of times. But life in rural villages, home to more than two-thirds of Indians, is now also being upended.

The first blow came in March, when Prime Minister Narendra Modi plunged the country into one of the world's strictest shutdowns on four hours' notice. Tens of millions of villagers working in the big cities lost their jobs. The flow of money back home, where it was a big chunk of the village's total income, ceased.

Then the migrant workers began returning. First it was a trickle, as hundreds of thousands illegally fled cities during the lockdown. It became a flood when the government allowed tens of millions to book special trains and buses as the lockdown eased.

Rajan Kumar, the head of a council that governs several villages in the northern state of Bihar, said about 1,000 migrant workers have returned, increasing the local population by more than 10% while intensifying competition for the few jobs in the area.

"The situation is grave," Mr. Kumar said. "Their incomes from the cities supported the education of their kids and well-being of their families in the village. Now how will they look after them?"

Nearly 900 million Indians live in rural areas, around two-thirds of the country's total population. They represent almost one-quarter of the world's rural poor.

Their share of India's population had been steadily declining for a generation as cities powered economic growth in an increasingly globalized world. Though city jobs were tenuous and urban life arduous, this migration of labor from the countryside led to a dramatic decline in extreme poverty. Only China, with its massive export manufacturing boom, has pulled more people out of rural poverty.

The pandemic has now thrown a massive wrench in the system, one of the ways the disease is reversing decades of gains against extreme poverty around the world.

The longer the economic contraction goes on and the deeper it gets, the larger the lurch back into poverty across the globe. According to a study at the United Nations University World Institute for Development Economics Research, between 80 million and 395 million more people could be thrust into extreme poverty if the global economy contracts between 5% and 20% this year, a range projected by international financial institutions based on various pandemic responses. Almost half of those additional poor could be Indian, according to the research.

In India, wages in poor rural areas such as the state of Bihar had been slowly increasing as migrant workers left for better-paying jobs in India's cities, and the money they earned circulated back into the local economy. The loss of their remittances and their subsequent return is now depressing local earnings, as the returned migrants compete for jobs as rickshaw drivers, street vendors and day laborers.

Rambilash Yadav, head of a multivillage council in a district of Bihar known as Begusarai, has wrangled with the return of about 90 residents to six villages. In the quarantine centers where the returnees were held, some demanded the meat and poultry meals they became accustomed to in the city, spurning the lentils and bread the village had to offer. Mr. Yadav worries what will happen as frustration over the local job market sets in.

"Village life won't be easy anymore," he said. "These youngsters are idling their time away right now, but in days to come there is going to be a serious crisis finding work in line with their skills."

He said the villages have launched a publicly funded project digging up a pond, but few of the returnees are signing up. "They feel it's a menial job with meager pay," he said.

Ramji Sahni, 39 years old, loaded marble and granite stone slabs for construction projects in the booming tech capital of Bangalore in southern India for almost 10 years. After returning to his village in Bihar, he was quarantined at a school before being allowed to join his family.

He is spending time with family and meeting friends in the village square, helping his wife grow vegetables in a small patch of family land. But the plot produces only enough to feed his family and won't replace the up-to-20,000 rupees he was earning each month in Bangalore.

The Indian government has tried to address the ballooning crisis by offering more low-wage jobs under a temporary employment program and revitalizing the rural economy by helping farmers escape a tightly regulated system that forced them to sell their crops at low prices through batteries of middlemen.

They can now sell directly to retailers and other buyers at market prices for nearly all crops. The government has also allocated money to build storage and processing facilities in rural areas, which should help prevent the sort of gluts that have driven prices down in the past.

Some returnees are determined to use the skills they learned in the cities.

Polus Khaka, a 32-year-old migrant who returned to Sarna Toli after 12 years working as a hotel manager in Mumbai, doesn't want to go back. He hopes to start his own hotel some day, but for now plans to set up a small roadside restaurant, hiring fellow returnees to work for him.

"I always believed Mumbai was a city where dreams come true," he said. "No more."

Still, new dreams and more government money likely won't be big enough to fill the hole created by the loss of income the migrants once earned in the cities, experts say. Overhauls to the agricultural sector, however significant, could take years to make a difference.

Meanwhile, both villagers and returnees struggle to adjust.

Isdor Lakra, 40 years old, returned to Sarna Toli recently after losing his job waiting tables at a restaurant in India's southwestern state of Goa some 1,200 miles away.

Unable to purchase a train ticket, he slept beside the highway for three days before finding a bus to Ranchi, the nearest city to his village. From there he squeezed into a flatbed truck with 15 other passengers amid potatoes and rice bags.

The journey home lasted seven days. Then he spent 18 days quarantined in a village school with other returnees.

When he finally returned to his home, "I ate like a pig and slept for a day and a half, then woke up to a fight outside the door," he said. "My neighbor was arguing with my father for letting me return and putting everyone's life in the village at risk."

He showed his neighbor a medical certificate he had obtained before leaving Goa stating he had no Covid-19 symptoms, and informed him about his quarantine in the village school.

"Nothing would calm him," he said. "'People go to the city to earn big money and run to the village when they starve' my neighbor kept saying."

Unable to find work, he has enrolled in the government's job program, helping move earth to build a dam. That earns him 200 rupees a day, about half what he earned at the restaurant in Goa.

Write to Vibhuti Agarwal at vibhuti.agarwal@wsj.com, Krishna Pokharel at krishna.pokharel@wsj.com and Rajesh Roy at rajesh.roy@wsj.com