Apple and Google are teaming up to allow contact tracing on many of the world’s smartphones. The partnership, which the companies announced today, will allow an estimated 3 billion people to opt in to location tracking through Bluetooth on their smartphones for the purposes of combating the Covid-19 pandemic.

Users will be able to receive alerts if they came into contact with someone who tested positive for Covid-19. The collaboration will require the world’s two biggest mobile operating systems, Android and iOS, to work together in a way that is unprecedented.

Both companies wrote in nearly identical announcements published on their respective websites:

Across the world, governments and health authorities are working together to find solutions to the COVID-19 pandemic, to protect people and get society back up and running. Software developers are contributing by crafting technical tools to help combat the virus and save lives. In this spirit of collaboration, Google and Apple are announcing a joint effort to enable the use of Bluetooth technology to help governments and health agencies reduce the spread of the virus, with user privacy and security central to the design.

The initiative will involve two steps. Next month, both Apple and Google will release APIs that allow contact tracing through third-party apps released by public health authorities. Users of both iOS and Android devices can download the official apps through their respective app stores.

Then, in the coming months, both companies plan on building an even broader contact tracing tool that won’t require users to download a third-party app. Anyone who chooses to opt in will then have their information shared with government health authorities and other contact tracing apps.

The use of such apps to track Covid-19 cases was first seen in China, and it has been applied to a less extreme degree in other Asian countries, including Singapore, Taiwan, and South Korea. The general consensus was that data privacy laws and cultural mores would prevent contact tracing from taking off in the West. That assumption has proved to be wrong, as governments in the US, UK, and EU have recently embraced the concept.

Robert Redfield, director of the US Centers for Disease Control and Prevention, told NPR this week that “very aggressive” contact tracing would be needed for America to resume normal life. In Europe, a group of scientists and experts from the tech industry is working on a framework for contact-tracing apps that will protect privacy. France, which is a member of that coalition, earlier this week announced it will launch a Stop Covid app for contact tracing. The UK’s National Health Service is working on such an app that will instantly notify users of close contacts with people who have tested positive for Covid-19.

Basing contact tracing on Bluetooth is meant to protect a user’s anonymity and privacy. Some nations, including China and Israel, have used GPS and other cellphone location data to monitor those with Covid-19, methods that raise stronger privacy concerns.

MIT researchers this week announced a system—which apps from public health authorities could use—that employs Bluetooth to automate contact tracing while protecting user privacy. Bluetooth signals generate random strings of numbers, which will be used as identifiers, instead of a person’s name. “I keep track of what I’ve broadcasted, and you keep track of what you’ve heard, and this will allow us to tell if someone was in close proximity to an infected person,” professor Ron Rivest, principal investigator of the project, told MIT News. But for MIT’s approach to work broadly, it would need the participation of Apple and Google’s mobile operating systems, reported TechCrunch, which it now has.

But privacy and civil liberty groups around the world have raised concern over how such technology, even during an unprecedented public health crisis, will fare in the hands of governments and Big Tech.

Kurt Opsahl, general counsel of the Electronic Frontier Foundation, wrote to Quartz: