Kentaro Iwata, professor at the infectious diseases division of Japan's Kobe University, described the situation on board as "completely inadequate in terms of infection control".
After visiting the ship, Prof Iwata posted a video to YouTube stating that the quarantine measures he witnessed failed to separate the infected from the healthy.
The expert said he was more afraid of catching the virus on board than he had been working in the field in Africa during the Ebola epidemic and in China during the Sars (severe acute respiratory syndrome) outbreak.