A father of three in Brooklyn is back to stockpiling medicine and rubbing alcohol. A publicist has put her plan to return to her office in Manhattan on hold indefinitely. And a mother in Central Park has again — and again — delayed taking her 15-month-old daughter back to the toddler music classes she loved.

“Big groups of kids, we’re not doing any of that,” said the mother, Aneya Farrell, 34. “She hasn’t seen a lot of babies over the past six months.”

As the city faces its first notable increase in coronavirus infections since a springtime surge killed more than 20,000, residents are again looking at their neighborhoods and wondering, after each rise in numbers, each passing siren: Is this a second wave?

The recent increase prompted Mayor Bill de Blasio and Gov. Andrew M. Cuomo to order lockdowns in several parts of Brooklyn and Queens where the infection rate has risen most sharply. The restrictions mostly affected neighborhoods with large Orthodox Jewish populations. But other neighborhoods faced partial lockdowns, including the canceling of indoor dining.

“It’s scary and upsetting,” Ms. Farrell said, “because we had such a good streak going.”

The increases have rattled many people in those neighborhoods and beyond, reminding them of the dark days of March and April when it was impossible to meet friends, eat out at a restaurant, go to church or visit parents.

Some New Yorkers see the rise in cases as a harbinger, the footfalls that announce an intruder’s arrival.

“I feel like the second wave is here — that same kind of doomsday feeling,” said Anya Ferring, 40, a fashion production consultant who lives in Far Rockaway, Queens, one of the neighborhoods experiencing a partial lockdown.

During the pandemic, Ms. Ferring has mostly maintained masked social distancing around her friends, with the masks gradually coming off in the summer. But recently, several of her friends have tested positive for the virus or have had to quarantine.

On a recent Saturday, she sat in Herbert Von King Park in Bedford-Stuyvesant, Brooklyn, with a friend, Kelly McKay, 39. Both wore masks.

“Summer is over, and the fun is over,” Ms. McKay said.

In several interviews in the past several weeks, city residents shared deep frustrations with their fellow New Yorkers who don’t appear to be following the same rules. The solidarity forged in the springtime outbreak appears, in some neighborhoods, to have fractured in the fall.

Michael Mitchell, a 49-year-old therapist from Washington Heights, said he was dismayed by what he saw on a recent subway ride. “People were not wearing masks — I asked them to put them on, and they wouldn’t,” he said. “So I feel like that sense of community living that we had is broken.”

Andre Williams, 45, the father who has resumed stockpiling acetaminophen and other supplies for his family in Clinton Hill, put it bluntly: “A lot of us took our eyes off the ball.”

Jennifer Burchette, a 29-year-old publicist from Bedford-Stuyvesant who changed her plans to return to her office, said too many people are acting as if the threat has passed.

“It’s not over because you’re over it,” she said.

In a survey of 1,000 New York City residents conducted in late September, 72 percent of the respondents said they expected a second wave of cases that will resemble the surge of April, according to the City University of New York Graduate School of Public Health and Health Policy, which released the findings on Wednesday. But beneath that fear were signs of optimism.

“In the face of this pandemic, it is encouraging that so many people continue to maintain their hope, a great sign of our resilience as New Yorkers,” said Ayman El-Mohandes, the dean of the school.

The recent increase in infections has led some of those New Yorkers who are staying put to make fast decisions in the face of a potential lockdown. Foremost for many were choices that involved schools.

The parents of more than half of the system’s 1.1 million students have opted to keep them home at least through November.

Others leaned hard the opposite way. Robert James, a single father of two children, said he planned to keep his 7-year-old son in school as long as possible. “I didn’t want my kid to have a full year out,” he said. “I thought if I didn’t put him in now, I might not get the chance, especially with a mayor with an itchy trigger to pull them out.”

He added, “It’s as safe as it’s going to be right now.” If the positivity rate rises to 3 percent and the city closes the schools again, “the kid stays home,” he said. (The current seven-day citywide rate is 1.7 percent.)

Many who endured the spring’s lockdown in the city are facing a possible second wave with an informed resolve to prepare, to have the things they wish they’d had then. For some, that means buying groceries and toilet paper in bulk. Others are visiting parents and grandparents while that still feels safe.

Erin Kommor and Keith White, both actors in Washington Heights, plan to get a dog to keep them company if they’re stuck inside. They said they monitor the positivity rates in daily emails from the city. “Not time to be stupid, I guess,” Mr. White said.

Others are consciously embracing the parts of the city they missed most during the last lockdown. Kitty Hatfield, 75, in SoHo, said she planned to get an insulated pad to use at her favorite restaurants, for as long as they’re open, so she can sit outside when the temperatures drop. Her plan: “Put more clothes on and keep doing what I’m doing,” she said.

Jessica Yan and Chris Uller have been traveling to Fort Tryon Park in Manhattan to find rock walls for climbing, with an urgency steeped in concern that at some point, they won’t be able to.

“I feel like a future wave is something that’s in the back of my mind, but I also feel like there’s a point at which we have to prioritize mental health as well,” Ms. Yan said.

Sezer Benoit-Savci, 17, who graduated from high school in the spring and is taking a gap year, showed up for his new job at the grocery-eatery Lea in Kensington, in an area where indoor dining is now banned. “Being shut down and locked in my house would be unfortunate,” he said.

Like the rock-climbers in Fort Tryon Park, Rachel Tigay, a 53-year-old high school social worker, has been focusing on exercise. She regularly meets friends for bicycle rides in the city. “I’m biking more than I ever have,” she said.

A sense of panic seems offset by experience: New Yorkers have lived through this before. Many of those interviewed expressed confidence in their ability to keep themselves safe, comforted in knowing that what they’ve been doing for months has worked so far.

“We’ll be watching the numbers, but it won’t affect our behavior because we’re already working from home,” said Ted Altschuler, a 57-year-old theater director who lives with Mr. Mitchell, the therapist. “We’re going to remain cautious.”

Joe Sigona, 73, of Kips Bay in Manhattan, recently sat reading a newspaper in Bryant Park — his “open-air library.” He watches the numbers, but plans to adhere to his simple strategy that has thus far seen him through: “Stay to myself, stay out of crowds, stay away from people,” he said.

Zoe Neuschatz, in Bedford-Stuyvesant, said she and her boyfriend had been mostly staying at home since the pandemic began, and they have no plans of letting up and have even considered stockpiling supplies.

“Maybe it’s my post-trauma stress from the spring,” she said. “I’m a nerd about it. I take it seriously, always.”