The National University Hospital (NUH) is recalling 178 paediatric patients - including 131 under the age of two years - who had been cared for by a nurse now confirmed to have tuberculosis.
They include 34 children who have received a transplanted organ. These children are on immunosuppression drugs and therefore at higher risk than normal children.
Associate Professor Daniel Goh, head of paediatrics at NUH, said the risk of any of the children contracting TB from the nurse is "very low", especially as she had donned a mask while working.
But the hospital is not taking any chances and is recalling patients who had been in ward 47 since July.
The nurse was treated at a GP clinic for her cough in July, but despite having a chest X-ray, was not diagnosed with TB. She was given antibiotics and seemed to get better.
How the disease is spread
Tuberculosis is an airborne disease transmitted through fine respiratory droplets from an infected person.
It usually affects the lungs but can also hit other parts of the body such as the brain, lymph nodes, kidneys, bones and joints.
Persons with TB are most likely to spread it to people who have close and prolonged contact with them, such as family members, friends and colleagues.
It is not spread through sharing drinks, utensils, cigarettes or kissing.
There were 1,454 new TB cases among residents here last year, 34 more than in 2013. In the past 30 years, Singapore had more than 1,000 new cases a year.
According to the Health Ministry, there has been a resurgence of TB cases in recent years.
TB can be cured with a combination of different drugs taken over six to nine months.
However, if the treatment is incomplete, TB germs may develop a resistance to the drugs.
Second-line TB drugs must then be taken for a longer period, but chances of a cure will be significantly reduced.
But the cough persisted, and last Wednesday she sought treatment again and was given a CT scan which showed a possible TB patch in her lung the size of a 50-cent coin. She told the hospital on the same day and was tested for TB. Last Friday, the results confirmed she had TB.
Fortunately, it is the normal and not multi-drug-resistant variety.
Dr Goh said his team spent the weekend trawling through the patient database to identify patients who might have had long exposure to her while warded, as well as those who are deemed at higher risk because of their age or disease.
Mr Joe Sim, NUH's chief executive officer, said: "We fully understand the anxiety of the parents and are taking this matter seriously."
Professor Paul Tambyah , a senior infectious disease expert at NUH, said TB transmission depends on the amount of exposure, the bacterial load of the carrier and the individual's immune system.
He said there is a one in 10 chance of people getting the bug if a person with TB coughs at them for two hours. Of those who get the bacteria, one in 10 would get the disease in his lifetime. This doubles for those with low immunity.
The first patients were at NUH for screening on Tuesday and more will be screened over the coming weeks.
They will have a chest X-ray to check for TB, and blood tests if aged five or older, and/or skin tests to see if they have the bug latent in them.
Any child diagnosed to have caught the latent bug will be given treatment, which has a greater than 90 per cent chance of preventing TB. For those with TB, chances of a cure are very high with treatment.
Madam Crystal Lim, 28, whose two-year-old son was in ward 47 for three weeks in August, was shocked when his doctor called to ask her to bring her son back for tests.
Her son had a liver transplant in October last year, but was back in hospital for treatment for a 3cm abscess on his buttocks. The doctor had explained that because of his transplant, he was at a higher risk.
She is worried, but not angry.
"I would be angry if the hospital did not do anything and the patients started getting TB. But they are doing something," she said.
As for the nurse, Madam Lim said: "She did not do it purposely. All the nurses look after the children very well. I hope the doctors can help her get well."
The nurse, who is on medical leave for two weeks, will be able to resume work as being on treatment means she is not contagious. Her colleagues in the ward have all been tested and found negative.
So far this year, 1,252 new TB cases have been diagnosed in Singapore.