https://www.reuters.com/article/us-health-coronavirus-tracing-explainer/explainer-what-is-contact-tracing-and-how-can-it-help-fight-the-new-coronavirus-idUSKCN21W2N3

Contact tracing has been used for decades to control the spread of infectious diseases. The basic idea is simple: track down infected people, then find everyone who has been near them and encourage those people to stay home until it is clear they are not sick.