Tencent is implementing a QR code system that tracks people who use public transportation. Anyone boarding a bus, taxi, or subway car needs to scan the code, linking their identities with that vehicle. And if they later turn out to be a patient with COVID-19, everyone who has shared a ride with them is notified.

It’s an imperfect system, one that backtracks to issue warnings rather than actively preventing the dissemination of the disease, but it has a precedent. Alibaba, employing its Alipay electronic wallet, which has 900 million users in China, has been assigning colors to people—green meaning clear for passage in public areas, yellow demanding seven days of quarantine, and red for 14 days of isolation.

So, whether you can leave a city, or even your apartment complex, depends on Alibaba’s algorithm.

But your color code is not determined by trained medical personnel. Often enough, the system flags people who aren’t exhibiting any symptoms and seem to be in perfect health.