WASHINGTON (AP) — President Donald Trump tweeted on Friday that four major cruise ship companies have agreed to suspend trips from the U.S. for 30 days, effective at midnight.

Trump said that Carnival, Royal Caribbean, Norwegian and MSC Cruises all agreed to the suspensions. He said the cruise line industry is a “great and important industry – it will be kept that way!"

The U.S. State Department advised Sunday against any travel on cruise ships, particularly for those with underlying health conditions. The advisory said the CDC has noted an “increased risk of infection of COVID-19 in a cruise ship environment.”

The cruise industry has taken a beating in the stock market since the coronavirus began to spread, and reports of people falling ill and being quarantined on ships surfaced.

Carnival had already suspended its Princess line of cruises for nearly two months. It said 18 ships that had cruises scheduled between March 12 and May 10 would be canceled.

Royal Caribbean announced the “voluntary” suspension earlier Friday, saying it would reach out to people who had already booked cruises.

Norwegian Cruise Line Holdings Ltd., which operates the Norwegian Cruise Line, Oceania Cruises and Regent Seven Seas Cruises brands, said it was suspending all cruise voyages embarking between March 13 and April 11.

MSC said it was canceling all its Caribbean cruises from Miami from March 14 through April 30.

Cruise industry leaders met over the weekend with Florida Gov. Ron DeSantis and Vice President Mike Pence and were asked to devise and fund ways to transport any passengers found to carry the virus. Cruise line heads agreed to enhance entry and exit screenings and establish shipboard testing, along with new quarantine standards. Some of the new protocols were expected to start taking effect early this week.

On Friday, shares of Carnival Corp. and Norwegian Cruise Lines posted double-digit gains. Still, Carnival's shares lost more than half their value and Norwegian shares a third of theirs over the course of the week. Their losses for the year to date are even steeper.

The cruise industry provided 1.2 million jobs and paid $50.24 billion in wages and salaries in 2018, providing an estimated $150 billion in global economic impact, according to Cruise Lines International Association, a Washington-based group that represents more than 95% of global cruise capacity.