There are seven teams comprising 10 people here, working in two shifts from 8.30am till 10pm, seven days a week. Their job - calling people to check if they are "close contacts" of coronavirus patients.

This group of contact tracers was activated when Singapore confirmed its first case of coronavirus infection last month.

Their job is a critical part of ring-fencing the virus by rounding up patients who may have been infected.

Close contacts are people who have had prolonged physical contact with or stayed in the same place as the patient.

It could include, for instance, someone driving the patient from the airport to a hotel, at a time when the person does not yet seem sick.

Any close contact must be quarantined for 14 days from their last contact with the patient.

But contact tracing does not start with these teams.

It begins in the hospital.

Dr Olivia Oh, assistant director in the Communicable Diseases Division of the Ministry of Health (MOH), said the hospital where the patient is warded would do an activity map - this refers to everything the patient has done and the people he has been with over the previous two weeks.

Her colleague, Mr Pream Raj, also an assistant director in the division, added: "The mapping is detailed, 24 hours, minute by minute, with no gaps."

If there are gaps, then the contact tracing team would call a patient to try and jog his memory. And if the patient is too sick, they would approach his next of kin instead.

He said although 14 days may seem like a long time ago, most people have routines, so it is not that difficult to recall what they had been doing.

If they had meals with people, and because of Chinese New Year many did, they need to remember who was there. If it was at a restaurant, they need to recall if any of the serving staff spent much time with them, or if the contact was casual.

For the 17 patients from China who were confirmed with the virus here, Dr Oh said they would start the contact tracing from the time they arrived in Singapore, assuming that they had caught the virus while still in China.

They would get the list of passengers sitting in the same row, as well as two rows in front and two rows behind the patient, from the airline, and get in touch with them.

Mr Raj said the airlines, taxi companies and Grab, since quite a number of the visitors use that service, have all been very cooperative - they are obliged to provide all information required under the Infectious Diseases Act.

Sometimes, patients have no taxi receipt, remembering only that they were in a blue cab. The contact tracers would go through the closed circuit television footage at the hotel driveway to identify the taxi.

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He added that they try to identify and get in touch with all possible close contacts of a patient to verify that what the patient had said was correct within 24 hours.

If the person is deemed at risk of infection, an MOH officer, together with a Certis guard, would serve notice of quarantine to the person, who will then be given the choice of staying at home or going to a government quarantine facility if his home premises are not suitable.

For home quarantine, the person must have a room and toilet that is not shared with anyone else.

The locations of where individual patients had been is also cross-referenced with other patients' activity maps to see if there are any overlaps.

These are analysed by another team and any links are flagged, and further cross-checks made.

Mr Raj said several such links have been found.

It could be anything from both patients attending the same event to meeting the same person.

Correction note: This article has been updated for accuracy.