https://techcrunch.com/2020/04/13/apple-google-coronavirus-tracing/

Last week, Apple and Google announced a partnership that will soon let users opt-in to a decentralized tracing tool, designed to help determine if a person has come into contact with someone who tested positive for COVID-19.

The opt-in system uses Bluetooth to transmit a randomized and anonymous identifier to nearby devices. A user can then choose to upload their anonymized data, which is then broadcast to other devices. If a match is found based on time spent and distance between nearby devices, a user will be told that they may have been exposed to a person — whose identity is not shared — with the virus.