By the time the first locally transmitted case of Zika was confirmed last Saturday, the authorities were already tracing past cases of fever in the area.

This allowed them to move swiftly on Zika testing of another 123 people last Saturday, said the Ministry of Health (MOH) and National Environment Agency (NEA) in a joint press conference yesterday.

Most of the results came in on Saturday night. As of noon yesterday, results were pending for five cases.

Said Professor Leo Yee Sin, senior consultant at the Communicable Disease Centre: "The minute the first case was made known - in other words, the blood was detected positive - a lot of investigations had already taken place to find out as much as possible."

The MOH was first alerted on the evening of Aug 22, when Sims Drive Medical Clinic flagged an unusual increase in cases of fever, rashes and joint pain.

The next day, MOH officials visited the clinic. The cases that the GP had seen were mild. The initial hypothesis was that a mild viral illness was being spread from person to person.

At that point, Zika was not specifically suspected.

The MOH told the clinic to refer any new cases to the Communicable Diseases Centre (CDC) for testing, and to start tracing past cases.

The MOH also told clinics in the vicinity and supervisors of a nearby construction site at 60, Sims Drive, to report any cases to the ministry.

Last Thursday, as part of tracing, the MOH approached the construction site's contractor for records of workers who had had fever.

That same day, a 47-year-old Malaysian woman developed a fever, rashes and conjunctivitis. She visited Sims Drive Medical Clinic last Friday, and was referred to the CDC.

The next day, it was confirmed she had Zika.

The MOH jumped into action that day, alerting the NEA and testing another 123 people for Zika, including 118 construction workers. The tests take three to four hours.

The NEA intensified anti-mosquito measures in the area, including inspections, misting and fogging, and increasing the frequency of drain flushing and oiling. These will continue for 14 days, with surveillance for another 21 days afterwards.

The NEA also started outreach efforts in the Aljunied Crescent and Sims Drive area.

Asked about the jump from one case last Saturday to 41 cases announced yesterday, Health Minister Gan Kim Yong noted that part of the reason more cases were discovered was because the MOH went back to past cases to "re-test, relook at the test results" last Saturday.

Of the 41 cases, 36 were found through the MOH's active testing of potentially infected people.

One reporter also asked him: "So it's not like you knew about it earlier and were keeping quiet about it?"

Mr Gan said: "No, of course not."