BAR HARBOR, Maine — On the day the virus arrived in Maine, Caroline Bloss lingered over pizza with her husband and father at Blaze, one of the few restaurants open in the offseason here. They were following the disease's progress closely as it shut down schools and sporting events further south. And now here it was in Vacationland, albeit in Androscoggin County, a couple of hours away.

“We’re not there yet, but I feel it’s coming,” said Bloss, who moved to Mount Desert Island in November after her husband took a job at a research lab here.

But it wasn’t just the virus that was threatening to show up. It was friends and family members who were already experiencing its ravages around New York and could think of nothing better than escaping to an island.

“Get ready for guests!” Bloss was hearing from them. “We’re coming up north!”

The idea that relatives, renters and summer residents might seek refuge here sparked debate about whether that was a good idea — at first in jest, but then fear began to emerge in Facebook posts. While some quickly set their concerns aside, other islanders considered closing the causeway that links Mount Desert Island to the mainland, voicing worries that have echoed in community meetings and chat rooms off both U.S. coasts and even in Hawaii.

Behind the anxieties was a question: If public health officials are recommending that people turn their families into islands, shouldn’t actual islands do the same?

“I’m getting 30 to 40 emails a day telling me to close down the ferry,” said Rick Hughes, a county council member across the country in the San Juan Islands of northwestern Washington state, about 90 miles from King County, which has one of the most serious coronavirus outbreaks in the country. “It can’t be those people and our people.”

Closing down the ferry would mean cutting off residents from food supplies and cancer care and myriad other resources for which they leave their treasured isles to “go to America.”

Descending as it did during the islands’ cocoon-like winter months, the coronavirus has transformed outsiders from an economic lifeblood to an existential threat. On Shelter Island — about two hours and 45 minutes from Manhattan — New Yorkers quickly bought up groceries. In Hawaii, some residents of the Big Island worry that sickened vacationers will overwhelm their hospitals. And six months after it was slammed by Hurricane Dorian, tiny Ocracoke, N.C., on the Outer Banks, is closed to visitors, leaving inhabitants wondering how it will ever recover if the virus deprives it of a vital summer tourist season.

“With Dorian, we could feel sorry for ourselves and rely on others coming to our aid,” said Amy Howard, manager of the Village Craftsman, which sells jewelry, pottery and an array of other handicrafts. “With this blow, we are going to have to work things out ourselves. We’ve done it before. . . . We can’t pretend we’re self-sustaining, because we are not.”

Thirty miles by sea from Mount Desert, the island of North Haven, Maine, population 375, declared visitors unwelcome — including its approximately 2,000 summer residents.

“People who do not reside on the island full-time may not travel to the island due to the significant increase in risk associated with the transmission of COVID-19,” the Select Board announced in a March 15 order.

North Haven quickly tried to put its welcome mat back out front, making a plea instead for “people to refrain from traveling away from the places where they currently live,” according to a letter sent by Rick Lattimer, the town administrator, to Maine Gov. Janet Mills (D).

Their conundrum reflects the fragile nature of these hamlets, sitting on “a piece of global real estate,” said Rob Snyder, president of the Island Institute. The year-round population is aging — on average almost 15 years older than the U.S. population — making it particularly vulnerable to this virus.

The threat of viral spread comes on top of the debilitating blow dealt to the lobster industry by President Trump’s trade dispute with China, which has exacerbated the wealth disparities between full-time and summer residents, who now own much of Maine’s prized coastal property.

“Any time you have stress, you see stuff like this,” Snyder said.

Islands have weathered pandemics in the past, sometimes by isolating themselves, which is often easier than it would be in a city or town on the mainland. During the 1918 flu outbreak, a few places survived by cutting off the outside world, according to John M. Barry, author of “The Great Influenza: The Story of the Deadliest Pandemic in History.” Among them were military installations on islands, including the naval training station that housed 6,000 people on the 120-acre Yerba Buena in San Francisco Bay.

One day before news arrived that influenza had hit San Francisco, the commandant ordered an immediate sequestration of the island: All personnel were required to remain there, and no visitors were allowed in, according to the Center for the History of Medicine at the University of Michigan. Recruits were placed in quarantine; even supply boats were kept 20 feet from sailors on the docks.

“No quarantine is entirely impermeable,” said Howard Markel, a physician and medical historian at the University of Michigan, who conducted studies into more than 40 communities. “These measures are not walls. You have to do more than one, like layers of Swiss cheese. If you do more than one, you can cover up the holes.”

Much as Mount Desert resident Erica Bunker said she would like to shut the island off, she knows it’s not realistic. She is cloistered in her home, worried that her asthma and her baby’s chronic lung condition put them at risk: She is concerned about “anybody coming to the island.”

New Yorker Allison Sullens and her family have self-quarantined in their Bar Harbor summer home, leaving only to visit the hospital and to be tested for the virus that already has sickened several friends.

“We do have a responsibility to lock ourselves in,” she said.

Ocracoke, on a tiny spit of land miles from shore, has embraced tourism and the summer crowds, hardly resembling the self-reliant fishing village Amy Howard’s ancestors settled three centuries ago, when William Howard served as quartermaster to the pirate Blackbeard.

“It’s not like we’re completely isolated,” she said. “We have to come and go.”

Today, Howard runs a gift shop and provides historical walking tours. The island has just one doctor and no emergency facilities. People take the ferry to the mainland for many things, from having babies to stocking up at Walmart.

And even though the Outer Banks is closed to visitors as pandemic prevention, modern-day marauders have shown up from another Outer Banks island after they heard the shelves at Ocracoke’s Variety Store were full.

“Once word got out, people from Hatteras came down to stock up and take stuff back,” Howard said.

In the Hawaiian archipelago, some residents fear sick tourists could overwhelm the state’s health-care system, arguing that the lack of consistency between the islands leaves everyone vulnerable.

The governor put out a message “encouraging our guests to postpone their vacations for at least the next 30 days and reschedule it for a later date,” and the mayors of Maui, Kauai and Oahu closed down bars and restaurants. But on the Big Island, Mayor Harry Kim (R) is leaving it up to individual owners to decide whether to continue catering to tourists.

The result, said Mouse Durgin, who moved to Hawaii with her husband 13 years ago, is that bargain hunters taking advantage of low airfares are still flocking to paradise.

Instead of being greeted with “Aloha,” Durgin said, some are being met with protest signs. “It’s not very fair to the tourists,” she said.

On Monday, Hughes, of the San Juan Islands, said council members asked the local health officer to order all hotels and transient lodging facilities to shut down.

“We are not open for visitors,” he said. But he also saw how hard it would be for an island to stop people from coming in. “I can pass a decree, but I can’t enforce it. I have 15 deputies to cover four islands.”

He said he hoped instead that people would find a common mission making masks and other supplies, and he choked up as he described a phone call he received from a local distillery offering to make hand sanitizer.

“How do we make people feel safe enough that they can focus on doing good?” Hughes said.

Yonatan Aldort, who was born on Orcas, one of the San Juan Islands, believes the current struggles reflect a message that resonates beyond the island’s rocky shores. He had been leading a public discussion about the detrimental effect Internet rentals are having on the community long before the coronavirus brought it to the fore.

“These islands, they are facing the first wave of what will face the entire world: too many people,” Aldort said. “As limited in space and resources as they are, islands are the first harbinger of that.”