She usually sees just a handful of patients suffering from gastric flu in a day.
But on May 16, when the number shot up to 28 patients, Dr Angela Cheong, 51, sensed that something was amiss.
Some of her patients were so sick they were throwing up in her clinic's toilets, said the Cheong Family Clinic general practitioner.
"I have been a GP for 20 years and I have never seen such a large number of patients on one day with the same type of symptoms," said Dr Cheong, whose clinic is in Owen Road.
"This was a lot more than I normally see. I was concerned about a bad viral outbreak."
Over a span of four days, from May 16, she saw 116 cases. So last Thursday, she alerted the Ministry of Health.
A joint statement by the authorities on Tuesday said that more than 180 cases of gastroenteritis were reported in the Owen Road area as of Monday. A large number either live or had visited the area and had also eaten at Pek Kio Market and Food Centre.
The centre was closed by the National Environment Agency for a thorough clean-up and disinfection yesterday. It will reopen tomorrow.
Cleaners were scrubbing the walls and giving the steam treatment at Pek Kio Market and Food Centre on 25 May 2016, after an outbreak of gastroenteritis cases in the area prompted a two-day closure of the centre. Photo: The Straits Times
One patient, Madam Kok Ah Lian, 65, who works at a coffee shop, said: "I didn't know so many people got sick. I thought it was just me."
She started vomiting last Thursday night and sought treatment the following morning. She was given medical leave and was back at work yesterday.
The youngest patient Dr Cheong saw was about a year old.
This led her to believe that the cause of the outbreak was not directly linked to Pek Kio Market and Food Centre.
"Babies don't eat food from the hawker centres. I have eaten food from the market and have had no problems," she said.
Most residents interviewed felt the same way.
"It's not the stalls. It's the environment," said a resident who wanted to be known only as Madam Tan, 45.
Madam Tan, who owns bakery Bread Connection in Owen Road, said she buys lunch from the food centre regularly and has not had any problems.
She blamed the large number of pigeons around the neighbourhood for the outbreak.
Another resident, who wanted to be known only as Madam Lam, 56, said her family was also unaffected.
The freelance draftsman said: "I usually have my morning coffee at the hawker centre and lunch there at least two to three times a week."
The authorities are still trying to determine the source of the virus.
Said Dr Cheong: "Medicine is about prevention and treatment. I have treated my patients, but I must prevent it from happening again. "That's why I alerted the authorities."
What is gastroenteritis?
Gastroenteritis is commonly known as stomach flu or food poisoning, said Dr Desmond Wai, a gastroenterologist at Mount Elizabeth Novena Specialist Centre.
He said symptoms of gastroenteritis include diarrhoea, vomiting and stomach pain. The illness can also be caused by bacteria infections, bacterial toxins, parasites and viruses.
A joint press release from the Health Ministry, the National Environment Agency, national water agency PUB and the Agri-Food and Veterinary Authority of Singapore about the increase in cases in the Owen Road area said that stool samples collected from those infected tested positive for rotavirus.
How is it transmitted?
Dr Wai said rotavirus is transmitted via faecal to oral transmission and was mostly likely introduced by someone infected with the virus.
He said: "It's likely that someone who had diarrhoea passed motion and did not wash his hands properly, and then prepared food." Rotavirus commonly affects children and is easily transmitted from person to person.
Dr Wai also said the virus might also be passed through contact with infected surfaces and contaminated food or drink.
How long can the virus survive?
Those infected can pass on the virus for up to 21 days and the virus might thrive better in moist environments like that of a market, said Dr Wai.
But Dr Wai said he was not sure how long the virus could survive on surfaces like chairs or tables.
"Disinfecting the whole market will definitely be good, but most importantly, you need to raise awareness among food handlers about good hygiene practices," said Dr Wai.
Is it life-threatening?
The virus is not life-threatening and patients almost always recover, said Dr Wai.
However, he said that patients who are very young or very old might face complications.
It is important for all patients to drink lots of water.
He said: "In this case, it might be hard to find the source (of the outbreak) because even after testing the stools of the food handlers, the patient might have already recovered."
How does one prevent it?
Dr Wai advised food handlers to take greater care with their personal hygiene.
"To prevent it from spreading, one should always wash one's hands thoroughly with soap and water after passing motion," he said.
"Also, if you're having diarrhoea, you should refrain from preparing food until you have recovered."
Goodwood Park Hotel's bakery licence was suspended on April 22 after its popular durian pastries were linked to cases of food poisoning. At least 76 cases were linked to the pastries.
Joint investigations by the authorities found that the outbreak was the result of poor personal and food-handling hygiene practices.
The suspension was lifted last Friday after the bakery was found to have satisfactorily implemented food safety measures.
About 230 people fell ill after eating food supplied by Kuisine Catering. The caterer's food was eaten at several events including a Chinese New Year gathering and a birthday party between Feb 12 and Feb 14.
Those affected suffered from vomiting, diarrhoea and fever, and five were sent to hospital.
According to the authorities, the outbreak was caused by a food-borne pathogen, Salmonella enteritidis, which is normally linked to raw poultry products.
The bacterium was most likely introduced as a result of poor personal and food hygiene practices.
The National Environment Agency banned food outlets from using freshwater fish in all ready-to-eat raw-fish dishes after an outbreak of Group B Streptococcus (GBS) infections linked to the dishes.
About 150 GBS cases were linked to the consumption of raw freshwater fish like toman and song fish, which are typically eaten with porridge.
In 2015, there were 355 cases of GBS infections and two deaths linked to the disease. This was an increase compared with 2011 to 2014, which saw about 150 cases annually.
The ban remains until further notice.
Additional reporting by Darryl Laiu, Lakeisha Leo, Mallika Sriram and Divyata Lalit Raut
This article was first published on May 26, 2016.
Get The New Paper for more stories.