Japan has been criticized by some medical experts who say their quarantine of a cruise ship with confirmed cases of the new coronavirus led to a further spread of the virus. But one of Japan's top health officials said he has documents from the Ministry of Health showing that most of the transmissions occurred before the quarantine began two weeks ago.

As of Wednesday, 621 people on the Diamond Princess cruise ship had been diagnosed with the COVID-19 disease. More than 500 passengers who tested negative for the virus were allowed off the ship Wednesday and more will follow in the next few days.

More than 75,000 people have been infected with the virus worldwide, and it has killed more than 2,000 people.

Dr. Shigeru Omi, Japan's community health director, defended the quarantine of the ship and said the new coronavirus is a "dishonest" disease because it's often transmitted asymptomatically.

Most coronaviruses are not contagious during their incubation period in a human host, Omi told CBS News correspondent Debora Patta, "but in this case, during the incubation period, they can infect others. The person with very minor symptoms or with very little symptoms can sometimes affect others. So this is a very difficult virus to deal with."

Omi said he thinks the isolation "was quite effective in preventing the spread of disease within in the ship."

More than 300 Americans onboard the ship were flown back to the United States on Monday. They are in quarantine around the country to prevent the spread of coronavirus in the U.S.



Cheryl Molesky, who was on the ship with her husband Paul, described the flight back to the United States with other evacuees as "truly horror."

"It was a nightmare," she said. "We were sitting with sick people."

Paul added that "the restrooms were behind that, so you had to walk by all the sick people to get your food or get to the restrooms."

The Moleskys are now at Lackland Air Force Base near San Antonio, Texas, where officials from the Centers for Disease Control and Prevention are monitoring their health.

Another cruise ship, the Westerdam, docked in Cambodia, is a concern after an American woman tested positive for the virus only after disembarking. U.S. officials are tracking more than 200 Americans who were on the ship and are now staying in hotels in Phnom Penh, and nearly 100 still on board.