The Department of Health and Human Services released a statement on Sunday saying one American has tested positive for hantavirus as the plane with 18 MV Hondius passengers is headed to the U.S.

Two of the passengers on the plane are traveling in the aircraft's biocontainment units "out of an abundance of caution," according to HHS.

The agency said that of the two, one passenger currently has mild symptoms and the other has tested PCR positive for the virus.

The plane is first going to the ASPR Regional Emerging Special Pathogen Treatment Center (RESPTC) at the University of Nebraska Medical Center before taking the passenger with mild symptoms to a second RESPTC, according to HHS.

"Upon arrival at each facility, each individual will undergo clinical assessment and receive appropriate care and support based on their condition," HHS said.

The plane carrying 18 passengers, including 17 Americans and one British national who is a U.S. resident, that disembarked from the MV Hondius has departed Spain and is headed back to the United States.

The American passengers who disembarked from the MV Hondius have arrived at Tenerife airport in the Canary Islands.

A total 18 people will be on the American repatriation flight, including the 17 Americans and one British national who is a resident of the U.S., the Spanish minister of health told ABC News.

-ABC News' Maggie Rulli

The 17 Americans aboard the MV Hondius have begun disembarking from the ship.

As of Saturday, none of the American passengers had tested positive for the hantavirus, according to the Centers for Disease Control and Prevention (CDC).

The aircraft that will return the Americans to the U.S. is expected to take off from Tenerife at 9:30 p.m. local time/4:30 p.m. Eastern time but could possibly depart earlier if circumstances allow, a source with the Spanish president's office previously told ABC News. It is not yet clear if or when that aircraft has landed in Tenerife.

The passengers will be flown to the National Quarantine Unit in Omaha, Nebraska, managed by Nebraska Medicine and the University of Nebraska Medical Center and the only federally funded quarantine unit in the U.S.

During a press briefing Friday, officials with the National Quarantine Unit said there is not yet a defined period for how long the passengers would remain at the unit.

-ABC News' Dragana Jovanovic, Aicha El Hammar and Maggie Rulli

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.”