https://www.wired.com/story/phones-track-spread-covid19-good-idea/

In 2011, two scientists at Cambridge University in the UK devised a clever way to measure and model the spread of the flu—an app called FluPhone that used Bluetooth and other wireless signals as a proxy for interactions between people, and asked users to report flu-like symptoms.

If you’d had lunch with someone who later got sick, FluPhone would let you know. Besides slowing the spread of the flu, the app promised to help health authorities monitor and model the spread of influenza. FluApp made headlines and the front page of the BBC website at the time. But in the end fewer than 1 percent of people in Cambridge signed up to use it.