https://www.techdirt.com/articles/20200330/07071644196/covid-19-will-someday-fade-away-wireless-location-data-practices-being-embraced-to-track-it-probably-wont.shtml

Location data has long proven to be hugely profitable to wireless carriers, given it's used by everyone from city planners to marketing departments. Now it's proving useful to help spread the track of COVID-19, allowing researchers to see not only who an infected person has been in contact with and where they've been, but also helping them predict where hot spots might appear next. Such technology was used during the Ebola outbreak in West Africa to help both track and predict the movement of the disease.

Now the government says it's working in partnership with the advertising industry to use such location data here in the States. The Wall Street Journal (paywall, alternative read at The Verge) indicates the Centers for Disease Control and Prevention and state and local governments have already received cell phone data about people in areas of “geographic interest." In this case, to track movements and determine how well people are adhering to "stay at home" restrictions: