Fifteen passengers remain in the quarantine unit and one person remains in the biocontainment unit at Nebraska Medicine, hospital officials said.

The passengers are undergoing in-depth interviews to trace their contacts as well as regular symptom monitoring, officials said.

Two residents of King County, Washington, are being monitored at home after potentially being exposed to hantavirus, the Seattle and King County Public Health Department said.

The two residents sat on a plane near an ill passenger from the cruise ship; that passenger was removed from the plane and later tested positive, officials said.

The two residents don’t have symptoms and are being monitored, officials said.

"Currently, no one in King County has symptoms of hantavirus and there are no cases of the virus in King County," the health department said. "The risk to the public remains low."

A resident of King County is also among the cruise ship passengers now being monitored at the quarantine center in Nebraska, the officials added.

Twenty-five crew members and two medical staffers remain on the MV Hondius cruise ship, which is en route to the Netherlands, Oceanwide Expedition said.

No one on board has hantavirus symptoms but the “medical staff will continue to monitor throughout the remainder of the voyage,” Oceanwide Expedition said.

The ship is expected to arrive on Sunday or Monday. The crew members will follow quarantine procedures when they disembark, Oceanwide Expedition said.

The French cruise ship passenger who tested positive for hantavirus “is presenting a severe form and is currently in intensive care in a serious condition,” French Health Minister Stephanie Rist said in French at a news conference on Tuesday.

She "has the most severe form of cardiopulmonary presentation, and she is receiving what is called, or rather what is the final stage of supportive care: extracorporeal circulation for artificial oxygenation," Risk said. "She has a lung-machine, a blood diversion, which we hope will allow her to get through this difficult period while her lung, which is being attacked by this virus and the damage to the vascular wall, recovers."

The other four French nationals who were taken off the ship were hospitalized for monitoring and they tested negative and "are doing well," she said.

Rist stressed that only people who were on the ship have tested positive.

Through contact tracing, Rist said “eight French nationals were identified on the flight between Saint Helena and Johannesburg. A second flight between Johannesburg and Amsterdam was carrying French nationals with a lower level of risk, in so far as a patient was present but was rapidly disembarked, which limited the exposure of travelers.”

“In total, 22 contact cases have been identified in France,” she said. “They have all been contacted, tested, hospitalized or are in the process of being hospitalized and are subject to rigorous health monitoring.”

-ABC News’ Othon Leyva

In the coming days, as the cruise ship passengers decide whether to stay at the University of Nebraska quarantine facility or self-monitor at home, those who opt to go home will not fly on a commercial flight, an official from the Centers for Disease Control and Prevention told ABC News. Passengers who opt to go home will have been assessed by health officials for any signs of illness.

-ABC News’ Youri Benadjaoud