The US State Department issued an alert Sunday urging US citizens “particularly travelers with underlying health conditions” not to travel by cruise ship due to the risk of coronavirus infection. The warning comes as 21 people on the Grand Princess cruise ship off the coast of California have tested positive for the coronavirus, according to the New York Times.

The alert from the State Department says many countries have put into place strict screening protocols that have denied entry to ships and prevented passengers from disembarking, all to try to contain the spread of COVID-19, the illness caused by the coronavirus.

“While the U.S. government has evacuated some cruise ship passengers in recent weeks, repatriation flights should not be relied upon as an option for U.S. citizens under the potential risk of quarantine by local authorities,” the alert states.

The Grand Princess cruise ship will dock at the Port of Oakland, California, on Monday, Bloomberg reports. Trump administration officials have said the passengers will be quarantined, with those who are ill receiving medical care at area hospitals.

President Trump said Friday he would prefer the passengers stay on board the ship. “I don’t need to have the numbers double because of one ship that wasn’t our fault,” the president told reporters at a press briefing at the Centers for Disease Control in Atlanta.