TOKYO — Hidden away in her cabin, Masako Ishida reckons she doesn’t face much risk from the coronavirus that has stalked her cruise ship, forcing an onboard quarantine of 3,700 passengers and crew members in the port city of Yokohama.

Ms. Ishida, 61, is trying to see the bright side as she faces two long weeks stuck inside the ship. She has a window to gaze out of, unlike some passengers. She, like her two traveling companions — her husband and her mother, both in their 80s — is healthy and not in need of any medications.

By no means, though, is this going to be easy.

After the virus struck, confusion came quickly. Ms. Ishida said she first learned that her two-week cruise might be extended not from the crew, but from an article she read online on Monday. That report said a man from Hong Kong who had disembarked from the ship, the Diamond Princess, in his home city on Jan. 25 had tested positive for the virus.