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Developing: On Friday, Apple and Google said they were building software into smartphones that could tell people if they were in recent contact with someone who was infected with the virus.

Some countries are using smartphone location data and other personal information to track coronavirus outbreaks or make sure people are staying home. There’s debate among public health experts in the United States about whether and how to follow their lead.

But this isn’t a black-and-white choice between personal liberty and saving lives. Not all citizen surveillance actually works in spotting or stopping illness. Even effective technology requires us to test many more Americans and have health care officials follow up.

I spoke with my colleagues Natasha Singer and Jennifer Valentino-DeVries about the benefits and limits of snooping to fight this pandemic. Remember to ask two questions about any proposed technology: Does it work? Is it creepy?