Hundreds of Americans who had been passengers on a coronavirus-stricken cruise ship in Japan landed in the U.S. on Monday, including 14 people who tested positive for the virus and were allowed to join the evacuation operation at the last minute.

The number of virus cases aboard the Diamond Princess ship rose to 454—about one in eight of those originally on the ship—prompting more questions about whether Japan made the outbreak worse by confining potential virus carriers in a narrow space that wasn’t designed for a long-term...