To enter many offices, restaurants, parks or malls in China nowadays, people must show their status on an app that determines whether they are a coronavirus threat.
There is often a moment of tension before opening the app on arriving at a location. A green light lets you in anywhere. A yellow light could send you into home confinement.
The dreaded red light throws a person into a strict two-week quarantine at a hotel. Such controversial use of technology has raised alarm in Europe as countries including Britain, France and Switzerland look into launching their own apps to trace infections.
But use has mushroomed across China, where the government keeps a close eye on the population and collects troves of personal data.