U.S health officials are turning to cell phones to help slow the spread of infections,as new corona virus cases explode nationwide.
Thanks to technology available on Apple and Google phones, you can now get pop-up notifications in some states if you were close to someone who later tested positive for Covid-19. The alerts come via state health department apps that use Bluetooth technology to detect when you (or more precisely, your phone) has been in close contact with an infected person's phone.
While these apps can't keep you safe -- they only let you know after you've been exposed -- they could prevent others from getting infected if you take precautions, such as self-quarantining, after receiving an alert.

Millions of people are signing up, although these apps aren't yet available in many states. Health officials believe the alerts could be especially helpful in cases where an infected person has been in contact with strangers -- for example in a bus, train or checkout line -- who wouldn't otherwise know they were exposed.

How the notifications work
iPhones and Android devices contain constantly changing anonymous codes that ping nearby phones via Bluetooth -- a process that starts once the user opts to get the notifications.
For the exposure notifications to be effective, Android users must turn on Bluetooth and download their state's Covid-19 notification app. On iPhones, the system is already baked into settings, although users must go to exposure notifications and make sure availability alerts are on.

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https://edition.cnn.com/2020/11/26/health/states-coronavirus-exposure-notifications-trnd/index.html