Their cases have helped to reveal the transmission pattern of the deadly virus, which has infected more than 20,000 people in mainland China, killing over 400, and spread to more than 20 countries.

It is known to be spreading primarily between people in close proximity, through respiratory droplets – such as saliva or mucus via coughs and sneezes – or by physical contact with viral material followed by its oral ingestion.

Of the five people in these Tianjin cases, the first two who sought medical help were a couple, of whom the woman was a salesperson in the store’s electronic appliances department. She had a fever on January 22, then her husband, who had not visited the store, had diarrhoea, also a symptom, two days later.

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The next person was another member of sales staff who worked on the same floor as the first but had not had close contact with her. She showed symptoms on January 24, after leaving town for a business trip on January 18, during which she met a person who health experts have said had a fever.

Scientists previously thought this third Tianjin person was the original virus carrier, although her link to the couple was not fully explained.

However, Zhang Ying, a section head at Tianjin Centre for Disease Control and Prevention (CDC), said in a press briefing on the matter on Sunday: “The place this third case visited outside Tianjin also has infected cases recorded, but there is no confirmation whether the person this salesperson met was a confirmed case of the virus or not, because that person did not seek medical help. We therefore were not able to confirm the origin of the third case’s infection.”

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Then a fourth person, also a salesperson at the department store, who sold shoes on the same floor, sought medical help. This patient, too, had left Tianjin, on January 12 to buy stock, and had shown symptoms of infection as early as January 21.

Health experts said this fourth person may have been infected on that trip, because her destination – which the experts did not disclose – had recorded confirmed infections. Nonetheless, they could not yet see a connection to the two other colleagues who became infected.

It was only when a fifth confirmed case related to the department store emerged on February 1 that the experts were able to map the spread of the virus among the five. The fifth person was a female shopper who spent three hours on January 23 shopping in the shoes and jewellery sections, on the floor where the third and fourth people were working.

Epidemiologists concluded that the shopper’s temporary exposure to the coronavirus showed it to be possible for the fourth person, bringing it back from her business trip, to have been the carrier who spread the disease to all three other women, the first of whom passed it on to her husband.

“Although the salespersons were working in different departments in the store, all the sections were on the same floor without any physical barriers separating them,” Zhang, the CDC official, said.

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“They were neighbouring each other and were less than four to five metres away from each other most of the time, and sometimes they would even be at a proximity of one metre. In such an environment, a visiting customer was infected in just three hours, so imagine how unavoidable the environment would have been for someone who works there eight hours a day.”

As of 6am on Tuesday, Tianjin had a total of 66 confirmed cases of the new coronavirus.