14/03/20: Correction: A previous version of this article stated that shares in Carnival had plummeted 53 percent this year. At the time of publishing, this was incorrect. The article has since been amended.

Berlin, Germany - Europe's multibillion-dollar cruise industry is in choppy waters as coronavirus fears swell, with customers abandoning holidays and governments becoming increasingly wary of allowing ships to dock if they have been to severely infected countries, or have suspected cases on board.

Europe is the world's second-biggest cruise market after North America, with around seven million passengers boarding liners in the Mediterranean and Baltic seas in 2018, according to the Cruise Lines International Association (CLIA).

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In recent days, several of the most renowned companies have halted some of their European operations, including Princess Cruises, Virgin Voyages and Saga.

On Friday, shortly after United States President Donald Trump declared a national emergency over the coronavirus pandemic, he tweeted: "At my request, effective midnight tonight, Carnival, Royal Caribbean, Norwegian, and MSC have all agreed to suspend outbound cruises for thirty days. It is a great and important industry - it will be kept that way!"

The industry was thrust into the spotlight in early February, after a coronavirus outbreak on board the Diamond Princess in Yokohama, Japan. Seven hundred people on board were infected with COVID-19 spreading through the ship's corridors during its two weeks of quarantine, leading to seven deaths.

The news caused a wave of cancellations and tanked stock prices.

Shares in Carnival, the world's largest cruise line, Royal Caribbean and Norwegian have all lost more than half of their value this year.

"The cruise industry has been hit particularly badly by [the coronavirus outbreak] and I would only imagine that it is going to get worse. We are seeing more people cancelling, cruises looking for alternative itineraries," Lloyd Figgins, a travel risk expert, told Al Jazeera.

"Within Europe, what's happened is there's been a knock-on effect from what we've seen in [Asia Pacific] and also the Americas," Figgins said.

Italy's dramatic decision to impose a nationwide lockdown earlier this week has effectively closed several of Europe's biggest cruise ports, tearing apart itineraries that were planned months or years in advance.

Liners bound for Venice, Naples and Civitavecchia, near Rome - which together take more than five million cruise passengers per year - will have to find new ports of call.

Unprecedented restrictions

Costa Cruises, an Italian line, said on Tuesday that passengers on current sailings would be able to disembark at Italian ports so they can return home, but all other departures in the Mediterranean would be cancelled until April 3.

Other companies have continued service, adapting to the rapidly changing restrictions, which have been haphazard and uncoordinated, ordered either by national or port authorities.

"There is hesitation about how to deal with things if there are cruise passengers on board with a suspicion of coronavirus," said Isabelle Ryckbost, secretary general of the European Sea Ports Organisation, which represents port authorities and administrations across Europe.

But cruise destinations will also have to weigh the health risks with the financial fallout, as many port cities have become economically reliant on the constant flow of tourists the industry provides in the summer months.

"I mean, not everyone is ill at the moment," said Ryckbost. "It's not affecting all regions at the moment. There are some cases in most of the European countries, but in some countries it's still very much contained."

To date, cases worldwide have topped 132,000, according to the World Health Organization. About 70,000 victims have recovered, according to Johns Hopkins University.

On Friday, the WHO described Europe as the new "epicentre" of the global pandemic.

Although cruise companies have been able to reroute Mediterranean journeys until now, this may become more difficult if more ports close and competition for space grows.

"If one port closes, but another is willing to accept [cruise liners], then you're going to get a massive influx of ships trying to get into that one port," said Figgins.

"And logistically, that's gonna cause a huge strain on local infrastructure."

Mounting travel bans add a further layer of unpredictability, as cruise passengers generally fly to and from their holidays.

Damage control

To reassure passengers, the CLIA, which represents 90 percent of ocean-going cruises worldwide, has issued sweeping restrictions and safety measures to be followed on ships.

Boarding will be denied to all passengers who have been to South Korea, Iran, China or Italy in the two weeks before the ship is due to set sail, as well as people who have had contact with anyone suspected or diagnosed as having COVID-19.

Temperature screening will also be introduced for all passengers.

"Travellers should know that their health and safety is the absolute priority for the industry," said Kelly Craighead, president and CEO of CLIA.

Several lines have also introduced new cancellation policies for customers, with Norweigan and Carnival offering full credit towards another cruise.

"The cruise industry has been so proactive in trying to make cruises safer by offering alternative itineraries if that's what their passengers want, and the hygiene side ... has always been exceptionally high and they're increasing that even further," said Figgins.

"The big problem here is the public fear that is currently going on, and I think that is something that is going to unfortunately penalise the cruise industry."

Health risks

Although cruises have grown more popular among younger people in recent years, around one in seven passengers worldwide is over 70, according to CLIA figures. This is the age group that the WHO says is more vulnerable to developing a serious illness from the virus.

In Europe, passengers from the biggest cruise markets such as Germany, the United Kingdom and Ireland are on average around age 50, according to CLIA.

While the European Centre for Disease Prevention and Control has yet to issue specific coronavirus health guidelines for cruises, the US Centers for Disease Control and Prevention has recommended that US citizens "defer all cruise ship travel worldwide" due to an increased risk of contracting COVID-19, especially among the elderly.

The warning, which faced strong objections from CLIA, came as the Grand Princess began to embark after several days of quarantine in Oakland, California, with 21 aboard testing positive for the coronavirus.

The possibility of further quarantines remains a risk for cruise travellers.

Passengers kept on board for long periods could expose themselves to coronavirus infection, according to mechanical engineer Qingyan Chen, who said that unlike the air filters on planes, those on cruise ships do not catch particles smaller than 5,000 nanometres.

The coronavirus is estimated, like severe acute respiratory syndrome or SARS, to be just 120 nanometres in diameter.

In addition to the virus spreading from person-to person transmission or via surfaces, cruise passengers could therefore be threatened through the ships' ventilation systems.

This would explain the spread of the virus through the Diamond Princess last month, even when passengers were confined to their cabins, Chen, who is also a Purdue University professor, told Al Jazeera.

"Although the concentration of the virus in the air is low, you're exposed to this low concentration for a long time, and therefore you might get infected."