TOKYO >> As the Diamond Princess cruise ship steamed back into port in Yokohama, Japan, Sunday morning after a night of quarantine at sea, passengers lucky enough to have windows and balconies could see fire trucks and 15 ambulances waiting for the ship.

It was an unnerving sign for the nearly 3,700 people who had been confined for six days on the ship, which has become host to the highest concentration of coronavirus cases outside China.

That afternoon, the captain announced over the intercom that six more people — five of them crew members — had tested positive for the virus. Eight others would be taken off the ship to be treated for unrelated medical conditions, the captain said.

“Now we will start counting ambulances and know that’s the number being removed,” said Sarah Arana, 52, a medical social worker from Paso Robles, California.

The six new coronavirus cases on the Diamond Princess brought the total to 70 since Japanese health authorities began testing people on the ship last week. New cases have been announced almost daily, and passengers have grown increasingly fearful that the quarantine — meant to protect people in Japan and contain the virus’s spread — could be putting them in jeopardy.

“I know that stress and anxiety compromise my immune system,” said Arana, who is on her first cruise. “My whole thing is just to stay calm because no matter what, I’m here. But every day it’s anxiety-provoking when we see the ambulances line up on the side of the ship.”

More than 2,600 passengers have been isolated in their cabins since the Japanese Health Ministry imposed the quarantine Monday, after discovering that a man who disembarked in Hong Kong on Jan. 25 had tested positive for the virus. More than 1,000 crew members are also quarantined on board, though they continue to provide services like meal deliveries.

“It’s sad to hear that there were another six cases,” said Tsutsui Masato, 70, a Japanese passenger who was on board with his wife. “I still don’t know how I should feel until we learn how the coronavirus is being transmitted.”

Some passengers said they could not understand why only a few hundred people on the 17-deck luxury ship had been tested for the virus — people who had had contact with the initial infected man or who have developed fevers.

“I do not now believe they are containing this epidemic by keeping us quarantined,” said Gay Courter, 75, an American novelist and avid cruisegoer who was isolated in a cabin with her husband, Philip Courter. “Something is wrong with the plan.”

With much still unknown about the new virus and how it is transmitted, Gay Courter, who once set a murder mystery on a cruise ship, said that even though the crew was working hard to protect passengers, there was no way of knowing if they were safe.

“Nobody can tell us for certain,” she said. “There’s no scientific evidence this is not being spread through food handlers or the people delivering the food, even in rubber gloves.”

Passengers have been speculating that the virus could be transmitted through the ship’s air ventilation system. Some shared their concerns with the U.S. Embassy in Tokyo.

On Sunday, the embassy sent all 428 American passengers a letter from a Centers for Disease Control and Prevention official, who said the center “has no current evidence to suggest that the virus spreads between rooms on a ship through the air-handling system.”

A spokesperson for Princess Cruises said the ship was equipped with a filtration system “that meets the standards and is comparable to those found in land-based hotels, resorts and casinos.”

Late Sunday night, Princess sent a letter to all passengers on board saying it would fully refund the costs of the cruise, including air travel and hotels, and that passengers would get a voucher for another cruise at a later date.

Another rumor had it that the U.S. government might try to take Americans off the ship before the 14-day quarantine period expired. A State Department official said that “current medical consensus and protocols state that the safest and most reliable way to prevent further spread of viral infections on cruise ships is for passengers to shelter in place, as the passengers of the Diamond Princess are doing.”

Once the quarantine ends, the official said, U.S. citizens can return home on commercial flights and will not be subject to additional quarantine periods.

On Wednesday, Carol Montgomery, 67, a retired administrative assistant from San Clemente, California, had a low-grade fever. Her husband John Montgomery, 68, a retired city planning director, was concerned about his diabetes and about whether he should clean the air ventilator he uses every night for sleep apnea.

“We’re sitting inside this room, and the number of cases is slowly rising,” he said. “It’s just very disconcerting that we can’t get tested to figure out if we have it.”

His wife eventually persuaded the ship’s medical office to let them leave their cabin for an examination. They were given flu tests, which came back negative, and Carol Montgomery, who had a urinary tract infection, was prescribed an antibiotic. They have not been tested for the coronavirus.

Given that passengers had expected to disembark last Tuesday, many with chronic health conditions like diabetes have been running low on medications. On Sunday, Health Minister Kato Katsunobu said in a television interview that medical supplies had been delivered to the ship for about 100 such people and that more would arrive later in the day for an additional 500.

On Sunday, the Montgomerys went out onto a deck for a prescribed fresh-air break — Carol Montgomery’s first time outside since the quarantine began. Under a pristine blue sky, the couple strolled in surgical masks, maintaining a 6-foot distance from other passengers.

A man in black shorts and a blue hoodie jogged by. “It feels great,” Carol Montgomery said.

Some passengers are frustrated by what they see as a lack of timely information. On Friday, they read in news reports — or heard from family and friends who were tracking reports online — that the number of cases on the ship had tripled.

“It was very upsetting to people to have their children and family members contacting them saying, ‘Oh my God, 41 more passengers tested positive,’” said Arana, who has been passing the time by drawing, testing out face masks she bought in Taiwan and taking an online course on herbal antiviral remedies. “So we were like, ‘We’re the last to know?’”

Many have been nervously reviewing their activities from early in the cruise, before the quarantine, and hoping they had not come into contact with the wrong person. Gay Courter thought about the meals, trivia nights and theater performances she had attended, including an opera that was staged the night before the quarantine was imposed.

“Every aspect of my perspective on everything we did has changed,” she said, “from ‘Boy, that was fun’ to ‘Why the hell was I there?’”