Why are cruise ships so dangerous when it comes to viruses spreading?

The quarantined ship Diamond Princess is pictured through barbed wire at Yokohama port. (AP)

Thousands of cruise ship passengers across the world have been quarantined in recent weeks after outbreaks of coronavirus onboard.

A total of 542 cases of coronavirus have been confirmed on the Diamond Princess, a cruise ship currently carrying 3,711 people outside the Japanese port of Yokohama.

While another ship with 3,600 passengers was also quarantined because of fears the disease may have spread widely among those onboard.

But why have cruise ships provided such a dangerous breeding ground for viruses?

Passengers being unloaded from another infected ship in Cambodia. (Getty)

Passenger behaviour partly to blame?

Professor Bill Keevil, from the University of Southampton, said he believes the passengers themselves are often unwittingly responsible for the spread of infection on cruise ships.

“The problem is that some potentially ill passengers, having looked forward to their holiday and spent a lot of money, do not want to miss out and board ship anyway,” he told Yahoo News UK.

“Alternatively, someone may innocently board the ship without any symptoms which subsequently develop onboard.

“They are required to immediately declare this and isolate themselves.

“The present coronavirus cases reported on the cruise ships are therefore not unexpected and isolating all the passengers in their cabins, while frustrating, is the best option until they can be taken off and housed in a more comfortable facility until the incubation period has passed.”

Passengers are seen on the deck of the Diamond Princess. (Getty)

Are cruise ships susceptible to outbreaks?

Professor Keevil argues the outbreak of illnesses on cruise ships is a well-known phenomenon within the industry.

He argues that more needs to be done to improve safety measures.

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“The cruise industry are well aware of the spread of disease onboard ship, considering that you can have hundreds or thousands of passengers in a relatively confined, isolated environment for days or weeks,” Professor Keevil added.

“They maintain regularly cleaning practices but still get occasional outbreaks of disease, particularly norovirus which is very hardy and highly infectious.

“They rely on the honesty of the passengers declaring if they are unwell or have had a recent illness as they board ship.

Which infections do cruise passengers catch?

Dr Connor Bamford, a researcher in virology at Queen’s University Belfast, said common infections spread easily on passenger ships because of the number of people living in close quarters.

He also said the conditions mean more dangerous infections like coronavirus are just as likely to spread.

“The viruses we worry most about are those than easily spread from one person to another and cause disease, such as influenza viruses or the novel coronavirus SARS-CoV-2. Viruses like these can be spread through the respiratory route,” he told Yahoo News UK.

“While some viruses, like seasonal influenza viruses, can spread by aerosols - very small drier droplets that can stay suspended in the air - most viruses are spread better in times of close contact that may bring infected people into closer and longer contact with uninfected people.

“This is very likely true of SARS-CoV-2.”

What do experts advise?

Dr Bamford said while it may be impossible to avoid “close contact” conditions on cruise ships, passengers can limit the risks by observing good hygeine.

“One of the reasons why transmission is more effective at closer contact is due to simply increasing the probability of spread but also by enhancing it as larger infectious droplets from coughing don’t spread far and land quickly on surfaces where they may be picked up unsuspectingly by uninfected people,” he added.

“One remedy for this is to practice good hygiene such as covering your mouth and nose when you cough or sneeze, and by thoroughly and regularly washing your hands.”