The COVID-19 crisis represents the most dangerous threat to public health since the H1N1 influenza pandemic of 1918. So far, the disease due to the SARS-CoV-2 virus has been countered with extreme measures at national level that attempt to suppress epidemic growth. However, these approaches require quick adoption and enforcement in order to effectively curb virus spread, and may cause unprecedented socio-economic impact. A viable alternative to mass surveillance and rule enforcement is harnessing collective intelligence by means of citizen empowerment. Mobile applications running on personal devices could significantly support this kind of approach by exploiting context/location awareness and data collection capabilities. In particular, technology-assisted location and contact tracing, if broadly adopted, may help limit the spread of infectious diseases by raising end-user awareness and enabling the adoption of selective quarantine measures. In this paper, we outline general requirements and design principles of personal applications for epidemic containment running on common smartphones, and we present a tool, called 'diAry' or 'digital Ariadne', based on voluntary location and Bluetooth tracking on personal devices, supporting a distributed query system that enables fully anonymous, privacy-preserving contact tracing. We look forward to comments, feedback, and further discussion regarding contact tracing solutions for pandemic containment.