This little QR code is a lot more powerful than it looks.
It helps collect data about your travel history, health status and more -- and China's using it to track citizens and stop those infected with the coronavirus.
First, you fill out a questionnaire. It asks for details like your body temperature. It then generates a color code.
And at checkpoints popping up across China - green means go.
(SOUNDBITE) (Mandarin) SUN YIMIN, 62, LOCAL SHANGHAI RESIDENT
"It's my first time to come outside after the epidemic but I already used the QR code several times and I think it's good. It's proof of your identity. Our country is upgrading and I think it's good to carry it out."
The codes are being scanned everywhere, from restaurants to apartment blocks.
Major cities and more than half of China's provinces have started using the color codes.
A way to make tracking down infected people easier for authorities.
But Western diplomats and activists have criticized China's mass surveillance in the past and expressed concerns about what else this data could be used for.
William Nee is the Human Rights Advisor at Amnesty International.
(SOUNDBITE) (English) BUSINESS AND HUMAN RIGHTS ADVISOR AT AMNESTY INTERNATIONAL, WILLIAM NEE, SAYING:
We've seen the use of big data to do predictive policing and detain people really for no reason. So there is a concern that although these data being perhaps collected for a legitimate purpose, that it could potentially eventually being misused in ways that we can't predict right now.''
It is unclear whether the data is analyzed on a national level as local authorities often use a variety of systems made by different companies, like Alibaba's Alipay and Tencent's WeChat.
In total, the virus death toll in mainland China has reached more than 3,000 with more than 80,000 confirmed cases.
But the number of reported cases has slowed in recent weeks.
China only reported four new locally transmitted cases on Sunday (March 15) which were all in Wuhan, where the virus outbreak originated.