The problem was not the girlfriend, the singer, the drummer and the guitarist told their recording engineer. They liked his girlfriend. They trusted his girlfriend. It was her four roommates and their friends and their lovers. Who were they seeing? What were they touching?

And so the members of Sure Sure, a self-described “art pop band” based in Los Angeles, made a proposal to their roommate and his girlfriend. She could live with them as “the ’rona” raged across the city, but there would be no more swinging by her place. It was all or nothing.

A few miles away, an actress was grappling with a similar predicament. She was living with a cousin who had started seeing someone before the coronavirus was declared a pandemic. Could the couple go for just one walk?

The actress, Lyla Porter-Follows, recalled measuring six feet of twine. Sure, she said, if they kept the string taut.

In New Zealand, Prime Minister Jacinda Ardern has been using the term “bubbles” to describe the groupings of people who continue close contact.

“Don’t pop the bubble” is a common refrain.

But sooner or later, most of us figure out that bubble maintenance is no simple matter. Some people still face concrete challenges: They have to go to work or share custody of children. Many report squishier quandaries. Should they let a friend’s friend into their bubble? How should bubble-mates set rules and expectations? Can they fight the urge to make exceptions?

The longer this goes on, the harder it will get, said Jonathan Smith, a lecturer in epidemiology at the Yale University School of Public Health.

“It will be easy to be drawn to the idea that what we are doing isn’t working and become paralyzed by fear, or to ‘cheat’ a little bit,” Dr. Smith wrote in a widely shared Facebook post. But “if your son visits his girlfriend, and you later sneak over for coffee with a neighbor, your neighbor is now connected to the infected office worker that your son’s girlfriend’s mother shook hands with.”

This, he added, is how a few people’s decisions undermine a promising public health intervention for us all.

The risks and benefits of three types of bubbles.

Connoisseurs of bubbles have observed at least three types.

The first is the Basic Bubble. It consists of whoever was living in a household pre-coronavirus — whether it was one person, a large family or a couple who want to break up.

The Basic Bubble can become a tense place when its inhabitants can’t agree on boundaries. A teacher in Salt Lake City said she hoped her three roommates would stay put. But two kept seeing people outside the household. Ultimately she and her top-floor roommate split with the first floor. They now communicate over FaceTime.

The next type you could call the Incorporated Bubble: people in one apartment or house invite friends, professional collaborators or relatives living in other households to move in with them.

Some do it because it seems like the safest way to continue to see people. Others do it to ward off loneliness. Soy Nguyen of Los Angeles, who moved in with a friend about a month ago, said this is why she’s still in good spirits. “Honestly, if we weren’t quarantined together and kind of lifting each other up, I don’t know how I would hold up.”

One challenge is that once you’ve started inviting other people into your bubble, it can be difficult to stop. Michael Nesmith, a high school football coach who lives with his wife and daughter in Maryland, had already agreed to let a niece and a cousin move in. Then his daughter asked if a close friend whose family lived far away could be a final addition.

“That was challenging,” Mr. Nesmith said. “I feel for this young man.” But he also felt responsible for the four women. “To run the risk of jeopardizing them for someone outside family, I just wasn’t going to do it,” he said.

Then there’s the Conjoined Bubble. The inhabitants of two or more bubbles get exclusive, agreeing to see one another while maintaining separate dwellings. Some people have practical reasons for this: they might agree to share home-schooling or babysitting with neighbors, for example.

Some expand their bubbles in this way because it seems fun and reasonably safe. But sometimes they have to reverse course when asking their friends to list every contact becomes exhausting and awkward.

Agreements vary. Ms. Porter-Follows’s four-person, two-household “isolation pod” committed to cook and take walks together, avoiding the store because one cousin has cystic fibrosis.

Sharon Needles, the Season 4 winner of “RuPaul’s Drag Race,” refers to her multi-household bubble as her “Corona-MySpace Top 8” and her “pirate ship.”

“We don’t venture outside the pirate ship,” she said. It includes her husband, two friends, her hairdresser and a three-person team that helps her produce drag shows that she broadcasts online.

One person in her bubble lives with a medical worker, which turns her Top 8 into something closer to a Top 100. Aside from that, bubble maintenance is easy, she said, because “we just don’t know or like other people.”

So which bubble is the safest?

Bubble size should not matter, researchers say, so long as the boundaries are firm. But, of course, with more people come more opportunities for leaks.

Some of these leaks — for example those created when a nurse or delivery person goes to work — cannot be avoided. But this is precisely why it’s so important to minimize them, said Steven Goodreau, a professor of epidemiology and anthropology at the University of Washington.

If two people in every household in a community of 200 people see just one friend, they will enable the mass spread of the virus, Dr. Goodreau and a team of modelers showed in a series of visualizations.

Staying inside our bubbles is like clearing highly flammable brush, said Carl Bergstrom, a biologist at the University of Washington who studies how infectious diseases spread.

“In a neighborhood where everyone’s house has a big cleared area with no brush in between, that fire can’t spread from one house to another.” When someone ventures beyond their bubble, they are scattering kindling, fueling the blaze.

Why social-distancing meetups rarely work.

Whatever kind of bubble people are in, the temptation to venture outside can be strong. You might feel as if your soul is about to break into a million pieces. Ellen Biscone, a grandmother who lives alone in Oklahoma City, has been there. Her children didn’t think it was safe to see her. Her solution? She drove to her daughter’s workplace and texted her, promising to stay in her car if her granddaughters came out.

“My soul was revived,” she said.

Oscar Castro of San Diego also employs the “drive-hi” to see friends without putting his parents at risk.

When we “cheat” on our bubbles, we are often driven by the belief that we can do it safely. But the promises we make ourselves about standing across the yard or waving instead of talking have a way of evaporating once we get there.

Autopilot seems to be one culprit, said Siouxsie Wiles, a microbiologist at the University of Auckland.

Though she is one of New Zealand’s most consistent communicators about keeping one’s distance, she said she recently found herself asking a stranger in a store to hand her a box of Corn Flakes from the top shelf. Deep-seated fears of offending others are also to blame; she suggests reminding friends that your concern is that you could infect them.

Dr. Goodreau said that years of public health work have taught him that people have a “good amount of wishful thinking” around friends and family, making it easy for us to convince ourselves that talking by the gate is not risky.

The only foolproof way to protect ourselves, researchers agree, is to avoid the situation altogether.

A Brooklyn woman, who did not want to be publicly linked to the coronavirus, learned this lesson recently while running with a neighbor. Before meeting up, they agreed to stay six feet apart. But once they started moving, it didn’t happen.

When she got home, her boyfriend was upset. On his own run, he had spotted them bopping along, practically touching. Why?

It was hard to articulate. “It seems like you can do it,” she said. “But actually trying to do it, it’s so difficult.”

She has not ventured outside her two-person Basic Bubble since.