One of 17 American citizens being repatriated from the hantavirus-hit cruise ship MV Hondius has tested mildly positive for the Andes virus, the US Department of Health and Human Services confirmed on Sunday.

A second passenger is showing mild symptoms but has not yet been confirmed as infected. Both are travelling to the United States in the plane’s biocontainment units as a precaution.

What is happening to the US hantavirus cruise ship passengers on their return?

The 17 Americans evacuated from the Canary Islands, where the MV Hondius made a stop, are being flown to Nebraska for assessment and monitoring. The passenger who tested positive will be taken to a specialized biocontainment facility.

The symptomatic but unconfirmed passenger will be taken to a separate, undisclosed location for further testing and care. Upon arrival, each person will undergo a clinical assessment and receive care based on their individual condition.

What is the Andes virus and how serious is hantavirus?

The Andes strain of hantavirus, identified as the cause of the MV Hondius outbreak, can cause severe lung illness that is fatal in up to 50 percent of cases, according to Arab News. Hantavirus typically spreads among rodents, and human infections are rare. Three passengers from the MV Hondius have died, while others have fallen ill during the outbreak.

The Department of Health and Human Services said both infected passengers were placed in biocontainment units on the repatriation flight “out of an abundance of caution.”

US health officials are expected to provide a further update on Monday after the passengers land. The remaining 15 Americans, who are asymptomatic, will also be taken to Nebraska for monitoring and assessment before being cleared to return home.