It was in October last year that the red flags were first raised.
Dr Cynthia Chee, a senior consultant at Tan Tock Seng Hospital's TB Control Unit, and her team realised that a patient diagnosed that month lived in a block of flats where several others had been diagnosed before with the same strain of multi-drug resistant tuberculosis.
When another case surfaced last month, she knew it was time to act.
"The significance was not clear to us at that time," said Dr Chee, about last October's discovery. "Then another case surfaced, and I just felt that it was something that MOH (Ministry of Health) should know about."
After more digging, it came to light that, altogether, six people in Block 203, Ang Mo Kio Avenue 3, had fallen prey to that particular strain of bacteria over a four-year period.
RAISING THE ALERT
The significance was not clear to us at that time. Then another case surfaced, and I just felt that it was something that MOH should know about.
DR CYNTHIA CHEE , a senior consultant at Tan Tock Seng Hospital's TB Control Unit, on the discovery last October of a similar TB case and a subsequent one in the same block of flats last month. MOH was informed then.
How this happened is still puzzling doctors as the patients came from four different households. Aside from three who were from the same household, the patients said they did not know or interact with one another.
The TB bacteria typically require close contact over a long period of time to spread.
Dr Koh Poh Koon, an MP for Ang Mo Kio GRC and a medical doctor, told reporters yesterday that Dr Chee was "very astute" to have noticed the pattern. "All of us see many patients on a daily basis, and the amazing thing is how she connected six patients over a four-year period."
Doctors were able to ascertain the cases were linked as the DNA fingerprints of TB cases in Singapore are recorded.
If the strains of two people match - as they did in the Ang Mo Kio case - then it is likely that they have been in close contact.
Fingerprinting makes it easier to track the spread of TB than the traditional method of contact tracing, or asking an infected person who he has been in contact with.
"It's very, very challenging," Dr Chee said. "People don't want to tell us about where they have been because there is a fear of stigmatisation from family or friends. Some are afraid they will lose their jobs."