https://arstechnica.com/information-technology/2020/04/apple-and-google-detail-bold-and-ambitious-plan-to-track-covid-19-at-scale/

In a bold and ambitious collaboration, Apple and Google are developing a smartphone platform that tries to track the spread of the novel coronavirus at scale and at the same time preserve the privacy of iOS and Android users who opt in to it.

The cross-platform system will use the proximity capabilities built into Bluetooth Low Energy transmissions to track the physical contacts of participating phone users. If a user later tests positive for COVID-19, the disease caused by the coronavirus, she can choose to enter the result into a health department-approved app. The app will then contact all other participating phone users who have recently come within six or so feet of her.