Together with the computer, the Internet and, increasingly, the Global Positioning System, the device has enabled health authorities to track disease outbreaks as they happen. Pioneers have used such systems successfully in developing countries that, lacking land lines, rely solely on cellphones to transmit data quickly from remote points. Voxiva, a mobile information technology company, deployed a disease surveillance system in India's Tamil Nadu state after the 2004 tsunami, and an HIV-Aids drug inventory network of 94 health centres in Rwanda in 2005. Using a similar system more recently, Indonesia started a pilot scheme to help speed up bird flu reporting. Singapore has vastly different conditions. Its compact urban setting and vulnerability to person-to-person or vector-borne transmission of disease as well as high phone subscriber rate make it a suitable environment to test how cellphone data can help stop disease transmission. For contact tracing, researchers will need to capture information that is usually regarded as private, namely, phone numbers and names of people a subscriber has called, so that exposure can be ruled out or preventive or curative action taken. This is a privacy concern not to be treated lightly.

A Harvard School of Public Health survey done shortly after the 2003 Sars epidemic found 38 per cent of respondents among international travellers 'very concerned' about possible privacy breach, although most would give airlines their phone number or e-mail address. Singaporeans would not be any less concerned about a more intrusive reach via the phone. They will expect adequate protection against abuse. Telephone service providers will need assurance against legal liability. But in the interest of preserving public health, the state should have sufficient legal justification to override a right that consumers should be prepared to suspend for their own good.