Scientists in Singapore may have found a clue as to how so many people aboard cruise ship the Diamond Princess ended up getting infected with COVID-19.
It appears the virus might be able to spread through air conditioning systems.
Just over a month ago, samples were taken from three isolation rooms at an outbreak centre in Singapore. Two of the rooms had their samples taken after routine cleaning - they consistently tested negative for SARS-CoV-2, the official name of the virus which causes COVID-19.
The third room however had samples taken before cleaning, and it tested positive. Thirteen out of 15 spots in the room had positive test results for the virus - including air outlet fans.
Three out of five spots in the bathroom - the toilet, sink and door handle - also tested positive.
"There was extensive environmental contamination by one SARS-CoV-2 patient with mild upper respiratory tract involvement," their study, published in the Journal of the American Medical Association, said.
"Toilet bowl and sink samples were positive, suggesting that viral shedding in stool could be a potential route of transmission. Postcleaning samples were negative, suggesting that current decontamination measures are sufficient."
The good news is that the air tests came back negative, suggesting the disease doesn't hang around in the air like the highly infectious measles does. But "swabs taken from the air exhaust outlets tested positive, suggesting that small virus-laden droplets may be displaced by airflows and deposited on equipment such as vents".
It's not clear from this study whether the viruses found in the air vents were viable and could have infected another person, as they didn't test it in the lab.
Many air-conditioning systems recycle air, including those used on cruise ships. More than 700 people on board the Diamond Princess got infected after crew and passengers were placed under quarantine at the start of February. Experts said this might have resulted in many more infections than would have taken place had people been allowed to disembark and be placed into proper quarantine facilities.
"A cruise ship with a large number of persons on board is more an incubator for viruses rather than a good place for quarantine," said Anne Gatignol, a microbiologist who studies viruses at McGill University.
Another expert told The Telegraph if SARS-CoV-2 is as small as the SARS virus which it's related to, it's far too small to be captured by cruise ship and building air filtration systems.
US President Donald Trump has asked for officials to implement a similar quarantine on the Grand Princess, which is currently parked off the coast of San Francisco. It's expected to dock on Tuesday (NZ time) in Oakland.
Planes operated by the major airlines have much more stringent filters. A number of airlines - including Etihad, Cathay, Qatar and Jetstar - have recently sought to reassure customers their systems can capture 99.999 percent of all viruses and bacteria.
"HEPA filters offer a similar level of performance to those used to keep the air clean in hospital operating rooms and industrial clean rooms," Cathay said.
Though it's likely to be an underestimate with many patients untested, there have been about 110,000 confirmed cases worldwide and 3802 deaths. New Zealand has five confirmed cases, no deaths, and no reports yet of community transmission.
New cases in mainland China - where the virus emerged - are becoming increasingly rare, with most new infections now being reported in Italy, South Korea and Iran.