Japanese expert who sneaked onto Diamond Princess cruise ship describes 'zero infection control' for coronavirus More than 620 people on board have tested positive for novel coronavirus.

A Japanese professor who gained access to the Diamond Princess cruise ship said in a YouTube video posted Tuesday that he was shocked by the lack of coronavirus infection control measures on board.

More than 620 people on the ship have tested positive for novel coronavirus, making the ship, which has been docked off the coast of Japan, the largest center of coronavirus infection outside of China.

"It was completely chaotic," said Dr. Kentaro Iwata, a infectious diseases professor at Kobe University in Japan, who reported witnessing passengers, crew members and medical professionals walking around the ship without wearing protective clothing.

One medical officer told Iwata that she'd given up protecting herself because she believed that she was already infected, he said.

"The cruise ship was completely inadequate in terms of the infection control," said Iwata, who has dealt with infectious outbreaks, including Ebola, cholera and SARS, for more than 20 years.

"There was no distinction between the green zone, which is free of infection, and the red zone, which is potentially contaminated by the virus," he said.

Iwata, who claimed he was kicked off the ship after his brief visit, had suggested several measures that authorities on the ship could take to improve infection control, including eliminating the informed consent papers that were being passed back and forth between passengers who were potentially infected with the virus and staff.

"We have to do something about this cruise," Iwata said. "We have to help people inside the ship."

Since leaving Diamond Princess, Iwata said he has voluntarily quarantined himself in a single room, separate from his family, and won't be going to work for several weeks.

"I'm very scared of getting the infection myself," he added.

ABC News' Joseph Simonetti contributed to this report.