I was just reading about China's "virus risk" app, and it's rather interesting (albeit rather authoritarian in its current form). It connects with people's AliPay accounts and sends the user's GPS coordinates to a central database, and if a person has been near a confirmed or suspected carrier, they're flagged as suspected.
It occurred to me that Google and Apple could easily implement a non-intrusive version of that. That is, only logging GPS coordinates locally. Only confirmed carriers get their location histories broadcast (names redacted). Everyone else's phones downloads the latest location reports and compares them against their stored location history. If they were possibly exposed, it could then warn the user and give them instructions.
If such a service ran on pretty much everyone's phone, you could stop local transmission of serious diseases in its tracks...