Samples from the two passengers, a Chinese couple, were sent for testing after three doctors and a nurse boarded the Costa Crociere ship in the port of Civitavecchia, north-west of Rome, to tend to a woman running a fever, local health authorities said.
Costa Crociere confirmed that its Costa Smerelda ship, carrying some 7,000 people including the crew, was on lockdown.
It said it a 54-year old woman from Macau "was placed in solitary confinement in the on-board hospital last night with her travel companion", and was following instructions from the health ministry.
The couple flew in to Milan from Hong Kong on January 25, before getting on the cruise, according to Italian media reports.
The Costa Smeralda, the company's flagship and the fifth-largest cruise ship in the world, "came from Palma de Mallorca and is currently engaged in one-week cruises in the western Mediterranean," it said.
The ship which is 337 metres long, has 20 decks and 1,628 cabins with balconies, had arrived in Rome from Palma de Mallorca, where its passengers disembarked for a walkabout in the city and had previously docked in Barcelona.
One Italian passenger who said he was on board the ship near Civitavecchia tweeted at midday on Thursday: "We're blocked on board the Costa ship without knowing the (official) reason."
"We're a bit worried of course. No-one is getting on or off the ship apart from the doctors. This holiday risks ending in a nightmare".
China reported its biggest single-day jump in novel coronavirus deaths on Thursday, as global fears deepened with at least 15 countries confirming infections.
The World Health Organization, which initially downplayed a disease that has now killed 170 in China, was preparing to meet Thursday to decide whether to declare it a global emergency.
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