How six people from four different households managed to pick up the same strain of multi-drug resistant tuberculosis (TB) is a mystery.
The six patients told Health Ministry officials that apart from the three from the same household, they did not know or interact with one another and had not congregated at common areas. They ranged in age from early 20s to 70.
The index case was a man who was first diagnosed with drug-resistant TB in February 2012. Another member of his household was diagnosed with the same strain three months later. That year, a woman living in the same unit was also diagnosed with a latent form of TB. This meant she would have shown no symptoms of the disease. However, in October last year, she too developed an active infection.
In May 2014, one of their neighbours was diagnosed with the same strain of TB. A fifth person was diagnosed in October last year.
When a sixth case came to the attention of a doctor at Tan Tock Seng Hospital's (TTSH) TB Control Unit, she noticed it was the same TB strain as that of another patient living in the same block, and notified the Ministry of Health (MOH).
The authorities declined to provide many details about the six, to maintain their privacy. However, they confirmed that two households were located on the same floor, with the other two some distance away.
Dr Jeffery Cutter, director of the communicable diseases division at MOH, said it was highly unlikely the patients could have caught TB by sharing the same lift, for example.
"It's possible, but we think that the likelihood is low," he said. "TB transmission requires close and prolonged contact." Even among members of the same household, the TB bacterium may take days or weeks to spread. It cannot be caught by touching a surface that an infected person has coughed on.
Professor Sonny Wang, director of the TB Control Unit at TTSH, said he believed there might have been contact between the six people which doctors could not identify. "We had some difficulty in getting people to divulge their daily activities to us, and they would tell us only the obvious things, like they went to work... and so on," he said.
"Maybe they were indeed not close contacts, and they were casual contacts, in which case we think that this is a very unusual situation because this is not the way TB ordinarily spreads."