US health officials said Tuesday they have issued quarantine orders for two passengers from the cruise ship at the center of a hantavirus outbreak who are now at a hospital in Nebraska.

The orders were signed by the Centers for Disease Control and Prevention’s acting director, Dr. Jay Bhattacharya, the CDC said in a statement.

The quarantined passengers are among 18 people who were aboard the cruise ship and are now being assessed in a special unit at the University of Nebraska Medical Center in Omaha.

The other 16 passengers have been asked to stay at the facility through May 31, according to the CDC.

Quarantine separates and restricts the movement of people who were exposed to a contagious disease to see if they become sick.

It’s an unusual step and it was not clear from the CDC’s statement why only two of the 18 passengers were ordered quarantined.

The World Health Organization said last Wednesday that a total of 11 hantavirus cases linked to the cruise have been reported, including three deaths.

Eight cases have been confirmed by laboratory tests.

Hantaviruses usually spread when people inhale contaminated residue of rodent droppings.

But the hantavirus that has caused the current outbreak, called the Andes virus, may be able to spread between people in rare cases.

The risk to the general public from the cruise ship outbreak is low, according to public health officials.