The doctors were puzzled. One after another, patients were coming in with exactly the same symptoms: fever, rash and joint pain. In most cases, the symptoms were mild, but the sheer number of patients exhibiting them took the doctors by surprise.
Dr Lim Chien Chuan, one of three general practitioners at the Sims Drive Medical Clinic, said he and his colleagues noticed a sudden spike in patient numbers around the second week of August.
Little did they know it at that time, but they were seeing the first known local transmission of the Zika virus in Singapore.
This came to light a few days ago because the doctors from this clinic called the Ministry of Health (MOH) for help in dealing with an infectious disease they could not pin down.
Given the rise in numbers they were seeing, they had suspected an outbreak of some kind.
Prior to that, the clinic would have at most three patients a day with these symptoms. From around mid-August, it started to get as many as 10 a day.
"We take turns (running the clinic), so it was very difficult to notice a trend," Dr Lim said. But the doctors consulted one another and realised all were seeing more such cases.
These three doctors, along with another friend, had joined forces to set up this clinic and another clinic in Jurong some 16 years ago. They are used to discussing cases with one another.
It was this habit that helped uncover the outbreak in Singapore.
It started with a message from Dr Tan May Yen asking her two partners for help in diagnosing one of her patients. The doctors realised they were all seeing more patients with the same symptoms.
Dr Chi Wei Ming said they started testing patients who consented "because we were seeing more than usual and they had exactly the same symptoms".
For some patients, they did as many as four different tests: for dengue, chikungunya, measles and rubella. Results for some, such as chikungunya, a viral infection also spread by the Aedes mosquito, took several days.
Alarm bells started to ring when all the test results for all the patients were negative.
Even then, the doctors did not think that they were Zika infections because none of the patients had recent history of travel to countries with the disease, and there was no known local transmission of Zika.
They contacted MOH on the evening of Aug 22 to ask for help as they could not explain the rash of patients with identical symptoms.
The next day, MOH officials were at the clinic, offering free consultations at the Communicable Disease Centre (CDC).
When offered the free consultations, some patients refused, but about five agreed to go, though the doctors do not know if all of them went.
One who did was a Malaysian woman, 47, who became the first to be identified as having locally transmitted Zika. That was on Saturday.
On getting the news, Dr Chi said they were "glad to know what was wrong with the patients, but on the flipside, it meant that Zika is now in the community".
By noon yesterday, the clinic had sent five more patients, including some foreign construction workers, to the CDC.
Another clinic in Sims Drive also sent three female patients and one male patient to the CDC.
Dr Lim said that now that Zika has been identified as the cause, the protocol is very clear. "We need to take blood and urine samples and send both samples and the patient to the CDC."
They call every time a patient has these symptoms, and an ambulance is dispatched to the clinic to pick up the patient and the samples.
The mystery has been solved, but the battle has just begun.