Details of 5,400 Singaporeans and 8,800 foreigners dating up to January 2013 have been compromised through a leak. In a country like Singapore with its homophobic government and lack of action on greater inclusivity, any individual known publicly to be HIV positive is likely to be devastated by this news.
Officials believe a 33-year-old US citizen who lived in Singapore from 2008 is behind the leak.
Mikhy Farrera-Brochez was convicted and jailed for fraud and drug-related offences in 2016 and was deported last year.
He is the former partner of Ler Teck Siang, the former head of Singapore's National Public Health Unit, who was convicted of helping Farrera-Brochez falsify his medical records to disguise his HIV-positive status.
Officials said Ler offered his own blood labelled as Farrera-Brochez's to allow him entry to the country.
The Health Ministry was aware of the leak in 2016 but assumed the police had retrieved all the information.
On Monday, health officials said they had tried to contact "virtually all" of permanent resident and Singaporeans on the list - but had only managed to speak to about 900.
A hotline has been set up for those affected, and counselling will be offered, Ministry of Health Permanent Secretary Chan Heng Kee confirmed.
He also said officials believe Farrera-Brochez was abroad, but do not know where.
"There is a risk of him continuing to publicly disseminate the info," Mr Chan warned.