The United Kingdom's Health Security Agency (UKHSA) announced on Friday morning there is “an additional suspected case of a British national on Tristan da Cunha,” while a Dutch KLM flight attendant who was thought to have it has tested negative for Hantavirus, officials said.

The total case suspected count remains at 12 after these latest developments.

UKHSA also said that “All British passengers and crew on board the MV Hondius will be asked to isolate for 45 days upon returning to the UK and UKHSA will closely monitor these individuals, with testing as required.”

-ABC News' Zoe Magee

The U.S. Centers for Disease Control and Prevention (CDC) activated its Emergency Operations Centers and has classified the hantavirus outbreak as "Level 3," multiple sources told ABC News.

That's the lowest level of emergency activation and is typical for this stage. It signifies the risk to the general public remains low, but the situation is being actively monitored by the public health agency.

Activating the Emergency Operation Centers indicates that an emergency team has been set up to support a response. Epidemiologists, scientists and physicians may have been re-assigned from their typical roles to assist with the response.

-ABC News' Youri Benadjaoud

One of the passengers aboard the cruise ship at the center of a cluster of hantavirus cases, says other passengers aboard the MV Hondius ship are remaining calm and keeping their spirits up.

In a statement provided to ABC News, passenger Kasem Ibn Hattuta said several doctors are now on board, which he said is reassuring to passengers, who are wearing masks and keeping their distance inside the ship.

"The vibe on the ship might’ve been calm with a bit of concern because we were worried about the sick ones, but now it doesn’t feel different than any of the other sea days we had before," he said in the statement. "Everyone is keeping high spirit[s], people are smiling and taking the situation calmly."

The statement went on to say: "We had several doctors join us on the ship before departing Cape Verde, which was also reassuring as we now have specialists on board, no one currently has any hanta related symptoms and we hope it continues this way."

He said passengers are free to share whatever they like online but some passengers have chosen not to post anything publicly because they want to maintain their privacy.

President Donald Trump, speaking to reporters on Thursday evening, said he has been briefed about the hantavirus cases tied to the MV Hondius cruise ship.

"It should be fine. We hope," Trump said.

He indicated his administration would be issuing a "full report" on Friday, but did not provide any further details.

“It's very much -- we hope -- under control. It was the ship, and I think we're going to make a full report about it tomorrow," Trump said. "We have a lot of people, a lot of great people, are studying it."

Asked if Americans should be concerned the virus may spread, Trump said: "I hope not. I mean, I hope not. We’ll do the best we can."

Fifteen passengers were welcomed to the University of Nebraska Medical Center’s quarantine unit Monday morning and one person who tested positive is in the biocontainment unit, according to officials.

The quarantine unit is more like a hotel, while the biocontainment unit is patient-based care, more like a hospital, officials said.

The 15 passengers at the quarantine unit are in “good spirits,” the unit’s medical director, Dr. Mike Waldman, said.

“We’ve been doing symptom monitoring, as well as temperature checks," he said. "Everyone here is asymptomatic and ... do not have a temperature at this time. They're all resting now and we'll do further assessments later in the day, once they've had a chance to sleep."

The one person in the biocontainment unit is doing well and does not have symptoms, but is “very tired” after a “really long journey,” officials said.

The 15 in the quarantine unit will have the option to stay in Nebraska at the quarantine center for the entire 42-day monitoring period, or go home to monitor symptoms, in coordination with their state and local health departments, officials said. This will be based on whether they develop symptoms, have a support structure at home to quarantine and can contact their health department, officials said.

Besides the 16 cruise ship passengers in Nebraska, two cruise ship passengers -- who are a couple -- were flown to Atlanta “for further assessment and care," officials said. At least one of the two in Atlanta had symptoms, officials said.

The two patients in Atlanta "are under medical evaluation" at Emory University’s Serious Communicable Diseases Unit, officials said, noting that their transfer there was "contingency planning."

"That means, if we're on it, since there was symptoms involved, they want to make sure that if they turn out to actually have the hantavirus, that it makes sure that the biocontainment unit here that provides medical care doesn't take up too much space, in case it's needed by the other passengers who are currently here in the more residential section," officials said.

The Georgia Department of Public Health said, "There is no risk to the public at this time. ... DPH is also remaining actively in communication with the CDC and other partners and will continue to do so for as long as necessary as we monitor each development in the coming days and weeks.”