As lion dancers snaked between conference room tables laden with plastic bottles, pens, notebooks and laptops, some staff from British gas analytics firm Servomex snapped photos of the performance meant to bring good luck and fortune.

But the January sales meeting in a luxury Singapore hotel was far from auspicious.

Someone seated in the room, or in the vicinity of the hotel that is renowned for its central location and a racy nightclub in the basement, was about to take coronavirus global.

Three weeks later, global health authorities are still scrambling to work out who carried the disease into the mundane meeting of a firm selling gas meters, which then spread to five countries from South Korea to Spain, infecting over a dozen people.

Experts say finding this so-called "patient zero" is critical for tracing all those potentially exposed to infection and containing the outbreak, but as time passes, the harder it becomes.

"We do feel uncomfortable obviously when we diagnose a patient with the illness and we can't work out where it came from ... the containment activities are less effective," said Dale Fisher, chair of the Global Outbreak Alert and Response Network coordinated by the World Health Organisation (WHO).

Authorities initially hinted at Chinese delegates, which included someone from Wuhan – the Chinese city at the epicenter of the virus that has killed over 1,350 people. But a Servomex spokesperson told Reuters its Chinese delegates had not tested positive.

Fisher and other experts have compared the Singapore meeting to another so-called "super-spreading" incident at a Hong Kong hotel in 2003 where a sick Chinese doctor spread SARS around the world.

The WHO has opened an investigation into the Singapore incident, but said its "way too early" to tell if it is a super-spreading event.

Scary and sobering

It was more than a week after the meeting – which according to a company e-mail included Servomex's leadership team and global sales staff – that the first case surfaced in Malaysia.

The incubation period for the disease is up to 14 days and people may be able to infect others before symptoms appear.

The firm said it immediately adopted "extensive measures" to contain the virus and protect employees and the wider community. Those included self-isolation for all 109 attendees, of whom 94 were from overseas and had left Singapore.

But the virus kept spreading.

Two South Korean delegates fell sick after sharing a buffet meal with the Malaysian, who also passed the infection to his sister and mother-in-law. Three of the firm's Singapore attendees also tested positive.

Then cases started appearing in Europe.

An infected British delegate had headed from the conference to a French ski resort, where another five people fell ill. Another linked case then emerged in Spain, and when the Briton returned to his home town in the south of England the virus spread further.

"It feels really scary that one minute it's a story in China ... and then the next minute it is literally on our doorstep," said Natalie Brown, whose children went to the same school as the British carrier. The school said in a letter that two people at the school had been isolated.

"It's scary and sobering how quickly it seems to have spread," said Brown.

Time running out

Back in Singapore, authorities were battling to keep track of new cases of local transmissions, many unlinked to previous cases.

Management at the hotel – the Grand Hyatt Singapore – said they had cleaned extensively and were monitoring staff and guests for infection but did not know "how, where or when" conference attendees were infected. The lion dancers, who posted photos of the event on Facebook, said they were virus-free.

"Everyone assumes it was a delegate but it could have been a cleaner, it could have been a waiter," said Paul Tambyah, an infectious diseases expert at National University Singapore. He added it was "very important" to find "patient zero" to establish other possible "chains of transmission".

But time may be running out.

Singapore health ministry's Kenneth Mak said the government will continue to try and identify the initial carrier until the outbreak ends, but as days pass it will get harder.

"We might never be able to tell who that first patient is," Mak said.

Meanwhile, the fallout from the conference continues to sow trepidation weeks after the event and thousands of miles away.

Reuters visited Servomex's offices in the suburbs of South Korea's capital, Seoul. It was closed and dark inside, and a building guard told Reuters employees were working from home.

A notice posted by building management stated a coronavirus patient had entered the complex, while several young women could be overheard in a nearby elevator discussing whether it had been used by the infected person.

"Do you think the patient would have gotten on this elevator or the other one?" one said.