A U.S. health official in a protective suit standing in front of a portable bio-containment unit speaks to U.S. passengers who have chosen to leave the Diamond Princess cruise ship, on a chartered evacuation aircraft to fly back to the United States, at Haneda airport in Japan February 17, 2020. Courtesy of Philip and Gay Courter/via REUTERS.

Health officials in protective suits on a shuttle bus transporting U.S. passengers who have chosen to leave the Diamond Princess cruise ship, to a chartered evacuation aircraft to fly back to the United States, at Haneda airport in Japan February 17, 2020. Courtesy of Philip and Gay Courter/via REUTERS. Image used for illustrative purpose.

Phil Courter, a U.S. passenger on board the Diamond Princess cruise ship, wears a face mask on a chartered evacuation aircraft to fly back to the United States, at Haneda airport in Japan February 17, 2020. Courtesy of Philip and Gay Courter/via REUTERS.

A bus arrives at the cruise ship Diamond Princess, where dozens of passengers were tested positive for coronavirus, at Daikoku Pier Cruise Terminal in Yokohama, south of Tokyo, Japan, February 16, 2020.

Buses believed to carry the U.S. passengers of the cruise ship Diamond Princess, where dozens of passengers were tested positive for coronavirus, leave at Daikoku Pier Cruise Terminal in Yokohama, south of Tokyo, Japan, February 17, 2020.

Buses believed to carry the U.S. passengers of the cruise ship Diamond Princess, where dozens of passengers were tested positive for coronavirus, leave at Daikoku Pier Cruise Terminal in Yokohama, south of Tokyo, Japan, February 17, 2020.

A bus believed to carry the U.S. passengers of the cruise ship Diamond Princess, where dozens of passengers were tested positive for coronavirus, leaves the Daikoku Pier Cruise Terminal in Yokohama, south of Tokyo, Japan, February 17, 2020.

A bus believed to carry the U.S. passengers of the cruise ship Diamond Princess, where dozens of passengers were tested positive for coronavirus, leaves the Daikoku Pier Cruise Terminal in Yokohama, south of Tokyo, Japan, February 17, 2020.

People wearing protectivie suits are seen near an airplane prepared for the U.S. passengers of the cruise ship Diamond Princess, where dozens of passengers were tested positive for coronavirus, at Haneda airport in Tokyo, Japan, February 17, 2020.

A passenger with his belongings is seen before the evacuation of the U.S. passengers of the Diamond Princess cruise ship, where dozens of passengers tested positive for coronavirus, as they arrive at Daikoku Pier Cruise Terminal in Yokohama, south of Tokyo, Japan, February 16, 2020.

Officers wearing protectivie suits are seen near an airplane prepared for the U.S. passengers of the cruise ship Diamond Princess, where dozens of passengers were tested positive for coronavirus, at Haneda airport in Tokyo, Japan, February 17, 2020. REUTERS/Issei Kato

U.S. passengers on board the Diamond Princess cruise ship, who have chosen to leave, are seen in a chartered evacuation aircraft to fly back to the United States, at Haneda airport in Japan February 17, 2020. Courtesy of Philip and Gay Courter/via REUTERS.

Officials are seen before the evacuation of the U.S. passengers of the Diamond Princess cruise ship, where dozens of passengers tested positive for coronavirus, as they arrive at Daikoku Pier Cruise Terminal in Yokohama, south of Tokyo, Japan, February 16, 2020.

A bus believed to carry the U.S. passengers of the cruise ship Diamond Princess, where dozens of passengers were tested positive for coronavirus, leaves the Daikoku Pier Cruise Terminal in Yokohama, south of Tokyo, Japan, February 17, 2020.

Packed suitcases and bags are seen on board the Diamond Princess cruise ship which has been quarantined since arriving in Yokohama, south of Tokyo, in early February after a man who disembarked in Hong Kong before it traveled to Japan was diagnosed with the coronavirus, February 16, 2020, in this picture obtained from social media. Mandatory credit PHILIP COURTER/via REUTERS.

Buses believed to carry the U.S. passengers of the cruise ship Diamond Princess, where dozens of passengers were tested positive for coronavirus, leave at Daikoku Pier Cruise Terminal in Yokohama, south of Tokyo, Japan, February 17, 2020.

U.S. passengers on board the Diamond Princess cruise ship, who have chosen to leave, are seen in a chartered evacuation aircraft to fly back to the United States, at Haneda airport in Japan February 17, 2020. Courtesy of Philip and Gay Courter/via REUTERS.

U.S. passengers on board the Diamond Princess cruise ship, who have chosen to leave, are transported by shuttle bus in Yokohama to Haneda airport to fly back to the United States via chartered evacuation aircraft, in Japan February 17, 2020. Courtesy of Philip and Gay Courter/via REUTERS.

A chartered evacuation aircraft operated by Kalitta Air waits for U.S. passengers who have chosen to leave the Diamond Princess cruise ship, to fly back to the United States, at Haneda airport in Japan February 17, 2020. Courtesy of Philip and Gay Courter/via REUTERS.

A worker in a protective suit checks the temperatures of passengers who were on board the Diamond Princess cruise ship, on a chartered evacuation aircraft from Japan's Tokyo International Haneda airport, en route to Lackland Air Force Base in San Antonio, Texas, U.S. February 17, 2020. Courtesy of Philip and Gay Courter/Handout via REUTERS.

Buses believed to carry the U.S. passengers of the cruise ship Diamond Princess, where dozens of passengers were tested positive for coronavirus, leave the Daikoku Pier Cruise Terminal in Yokohama, south of Tokyo, Japan, February 17, 2020.

Passengers display a banner reading 'Trump 2020' on the deck of the cruise ship Diamond Princess, where dozens of passengers were tested positive for coronavirus, at Daikoku Pier Cruise Terminal in Yokohama, south of Tokyo, Japan, in this photo taken by Kyodo February 14, 2020. Mandatory credit Kyodo/via REUTERS.

American passenger Gay Courter packs her belongings on board the Diamond Princess cruise ship which has been quarantined since arriving in Yokohama, south of Tokyo, in early February after a man who disembarked in Hong Kong before it traveled to Japan was diagnosed with the coronavirus, February 16, 2020, in this picture obtained from social media. Mandatory credit PHILIP COURTER/via REUTERS.