ROME—More than 7,000 people were quarantined on an Italian cruise ship in the Mediterranean port of Civitavecchia outside of Rome after a Chinese woman from Hong Kong came down with a fever and symptoms that mimic the coronavirus. Initial tests seemed to exclude the deadly virus, according to Italian media, but passengers were expected to be kept onboard overnight as a precaution. A second round of tests late Thursday completely excluded contagion, and Italy’s health authorities gave the green light for passengers to disembark. But as of Thursday evening, the mayor of Civitavecchia refused to authorize any passengers to get off the ship. Tourism operators, suddenly confronted with a new nightmare scenario tied to the virus, scrambled to address fears of an outbreak on a cruise ship. “We have no information, the internet inside the ship isn’t working, and we can’t get news. But above all we take meals together in the common areas, and we don’t know if someone is infected,” Liborio Iervo...