Eighteen Americans that had previously been on board Princess Cruises' Diamond Princess are infected with coronavirus.

Eleven of them are hospitalized in Nebraska. Five are under quarantine in Texas, as are another two in California, said Dr. Nancy Messonnier, director of the Center for the National Center for Immunization and Respiratory Diseases, said at a Friday press briefing by the Centers for Disease Control and Prevention.

The 11 hospitalized in Nebraska were part a group of 13 Diamond Princess cruise ship evacuees taken to the University of Nebraska Medical Center Monday after passengers arrived in the U.S. have tested positive for coronavirus, the medical facility confirmed Thursday night.

University of Nebraska Medical Center spokesperson Taylor Wilson said 10 people are currently in the facility's National Quarantine Unit and three others are in the biocontainment unit.

One man had already been transferred to the hospital’s biocontainment unit Monday because his symptoms included cough, fever, shortness of breath, lightheadedness and an undisclosed chronic condition that would make him particularly vulnerable to the COVID-19 virus.

"Most of our guests aren’t showing symptoms of the disease, however several others are exhibiting minor symptoms," Wilson said.

At a press conference on Monday, Dr. Mike Wadman, the co-medical director of the National Quarantine Unit, had said that the 13 passengers would be spending at least two weeks in quarantine in Omaha and that their confinement period would likely be extended in the event of a positive test.

'I feel ill-used by my country':Two groups of US Diamond Princess passengers, two disparate experiences

As of Friday morning, there were 634 confirmed cases of coronavirus that stemmed from the Diamond Princess, the most of any site outside of mainland China.

The new cases come one day after Japanese officials confirmed the deaths of two elderly Japanese passengers from the cruise.

According to Japanese broadcast outlet, NHK, the two Japanese cruise passengers who died from the virus were an 87-year-old man and an 84-year-old woman.

The story so far: Two Diamond Princess cruise ship passengers die from coronavirus; Westerdam crew cleared

The two passengers were taken to the hospital on Feb. 11 and 12, respectively, and each tested positive the day after they were admitted, Health Ministry official Masami Sakoi said. They are believed to have been infected before the ship was officially placed under quarantine on Feb. 5.

A health ministry official confirmed that the passengers had been previously hospitalized in serious condition and had existing chronic diseases. The official spoke anonymously, citing office protocol.

“Our hearts go out to the families, friends and all who are impacted by these losses," the company said in a statement provided by spokeswoman Alivia Owyoung Ender. "All of us at Princess Cruises, as well as the crew of the Diamond Princess, offer our sincere condolences.”

The Backstory:We asked one of the nation's top coronavirus officials some direct questions

Quarantine period ends, but new ones begin

Approximately 600 passengers disembarked the ship Wednesday after the Diamond Princess' official 14-day confinement period expired.

Health Minister Katsunobu Kato told new media the mass disembarkation into Japan of passengers from the ship is set to end Friday, while dozens of foreign passengers are flying back to their home countries on flights chartered by their governments.

However, more than 100 American passengers elected to finish their quarantine on the ship rather than leave on one of last weekend's evacuation flights. According to a letter from the Centers for Disease Control and Prevention, they must remain symptom-free and continue to test negative for two weeks before re-entering the United States.

And they aren't the only ones who are facing restrictions.

Most crew members and other passengers who have not completed their 14-day quarantines because they had more recent contact with infected people are staying on the ship for now but they will be transported to a government facility to be quarantined in isolation.

Japan is in discussions with Princess Cruises and home countries of foreign crew members over their future movements, Kato said.

Additionally, other countries are quarantining returning citizens – and some are testing positive for coronavirus.

Eleven Israelis returned home early Friday and were taken to the Sheba Hospital near Tel Aviv for quarantine and testing.

Australia said two passengers tested positive after they returned home. Because of the incubation period of the virus, more cases may be expected, said Brendan Murphy, Australian Chief Medical Officer.

"We don't know that (more will test positive), but if they do, we are completely well set up to detect and manage them and isolate them," Murphy said.

Kato said Australia, like the U.S., brought home a mixture of passengers who tested negative and others who were not tested and had an unknown status, therefore it was difficult to know when or how they had contracted the virus.

NIH official: Cruise quarantine was 'ineffective'

Japan's government has been questioned over its decision to keep people quarantined on the ship, given the tight quarters and the difficulty of isolating sick people from the healthy. Six government quarantine workers contracted the virus, raising questions about the protective measures used.

Other passengers who finished their quarantine on the ship are still testing positive for the virus, which has an average incubation period of 5.2 days, according to Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

David Abel, the British passenger who likened the quarantined ship to a "floating prison" in a Facebook post earlier this month, revealed Thursday that he and his wife, who booked the cruise to celebrate their 50th wedding anniversary, both tested positive for COVID-19 and are hospitalized in Fujisawa, Japan.

In a Monday interview with the USA TODAY Editorial Board, Fauci said that in hindsight, the Japanese government's decision to quarantine the 3,711 Diamond Princess passengers and crew on the ship was "ineffective" in stopping the virus from spreading.

"Something went awry in the process of quarantining on that ship, said Fauci, America's foremost authority on infectious disease. "I don't know what it was, but a lot of people got infected on that ship."

Kato, however, said that passengers who returned home on the U.S. and Australian flights did so before completing the Japanese quarantine process and that Japan's ongoing disembarkation of passengers is still adequate.

Coronavirus Q&A: What we know – and don't – about the COVID-19 outbreak

Contributing: Morgan Hines, Julia Thompson, David Oliver, Curtis Tate, USA TODAY, The Associated Press