NEW DELHI — As the United States and Europe grapple with fresh surges in coronavirus cases, the outbreak in India is slowing for the first time since the pandemic began.

Epidemiologists and doctors say the virus is in retreat — at least for now — in this country of more than 1.3 billion people.

After seven straight months in which cases increased relentlessly, culminating in a devastating September surge, the number of new infections per day in India dropped sharply in October.

India is home to one of the largest outbreaks on the planet. Last month, the country hit a peak of nearly 100,000 cases in a single day, a record in the pandemic. Since then, however, daily cases have fallen by about half and deaths by about a third.

The downward trend in India’s cases means it is no longer on track to overtake the United States as the country with the most coronavirus cases in the world. India has 7.7 million cases compared with 8.3 million in the United States. Each day this week, India has reported fewer new cases than the United States.

India is still adding more than 50,000 cases a day, an enormous challenge for its health-care system, especially in rural areas. The country is preparing to celebrate its biggest holidays, normally a time when large crowds gather.

A fresh spike in infections after the festival season — when colder weather also drives people indoors — is a real possibility, experts say. The winter months bring severe air pollution across a swath of northern India, which exacerbates respiratory illnesses.

Still, the recent downturn in India’s new cases is striking, particularly at a time when Europe and the United States are witnessing a resurgence in infections. India has increased testing dramatically but still conducts much fewer tests on a per capita basis than the United States. More than 115,000 Indians have died of the coronavirus, according to official statistics.

During a televised address Tuesday, Indian Prime Minister Narendra Modi urged citizens to remain vigilant. “This is not the time to be careless,” he said. Modi noted that in the United States and Europe, cases had declined but now “suddenly they are increasing again, and it is an alarming rise.”

Epidemiologists say the current decrease in cases in India is encouraging even if it may not last. “There is definitely some relief, but it is no time to celebrate,” said Giridhar Babu, an epidemiologist at the Public Health Foundation of India. “We might have these ups and downs for some time.”

The reasons behind the decline are not fully clear. India abandoned its nationwide lockdown in June and ever since has moved to reopen its crippled economy. Experts believe cases are falling now in India because the virus has infected a considerable swath of the population in its large, densely populated cities. Antibody surveys suggest that a third of the people living in Delhi have already been infected, as have nearly half the people living in slums in Mumbai.

The virus has “taken its own course,” said Jayaprakash Muliyil, a leading Indian epidemiologist. “The fodder that feeds the fire is susceptible people.” When that fuel is in shorter supply, he added, cases come down.  

Doctors and hospital administrators on the front lines of India’s fight against the virus say they have seen marked decreases in coronavirus cases in recent weeks. Vaishali Jadhav runs a private hospital in a suburb of Mumbai that was converted into a covid-19-only facility earlier this year. Just last month, she was scrambling to secure enough oxygen for her patients as virus cases surged in the region.

Since then, however, the number of patients has fallen by more than half. Now Jadhav plans to ask the local authorities next month if the hospital can resume treating people with other ailments.

She wasn’t willing to declare that the worst was over. “The patients have really reduced a lot, but we need to wait and watch,” Jadhav said.

Giri Babu Nadella, a doctor on the other side of the country in the southern Indian city of Visakhapatnam, witnessed a similar decline. His hospital, too, became a covid-only facility in July and struggled with an acute shortage of medical staff: At one point, the hospital had only four doctors and Nadella said he attended to nearly 100 patients by himself.

By the start of September, the hospital was full, with 400 coronavirus patients in total, Nadella said. Now it has only 80, and Nadella is preparing to reopen admissions for other types of patients.

The timing of the decline in cases caught experts by surprise. Bhramar Mukherjee, a biostatistician at the University of Michigan who studies India’s outbreak, said she began to see transmission rates falling in many Indian states. In the state of Maharashtra — the hardest-hit in the country — new cases dropped from more than 24,000 a day in mid-September to 9,000 at present. Some states have seen cases rise in recent weeks, including Kerala, which received plaudits for its early success in controlling infections.

The model Mukherjee and her team developed predicts India will still reach 10 million cases by the end of the year. She is hopeful that the declining trend will continue for some time but warned that fresh surges probably lie ahead. “I do expect that there will be another peak,” she said.

While the virus has spread throughout the country, epidemiologists warn that large numbers of people remain susceptible, particularly in rural areas where the vast majority of Indians live. Testing is far less prevalent in such areas and health-care infrastructure is feeble.

Babu, the epidemiologist at the Public Health Foundation of India, also worries that some people who have refrained from going out in the pandemic — including elderly Indians — could be exposed as the country fully opens up. Babu likened uninfected pockets of communities to a warehouse full of firecrackers. “You just need somebody to light it up,” he said.