The term "contact tracing" means what it sounds like: it's the process of tracing the contacts of an infected individual, in hopes of reducing the spread of a disease.
With COVID-19, a proportion of infected individuals are asymptomatic, meaning they can spread the disease but do not experience any symptoms themselves. This population of infected individuals can cause great harm in terms of spreading the disease, as they may not know that they carry the disease and may not adhere to social distancing guidelines with the degree of seriousness with which they should. With the enforcement of contact tracing, ideally all people who have been in contact with an infected individual would be notified of the infection. Even if the individual is an asymptomatic disease carrier, he or she should be more cognizant of the possibility of carrying the disease. This should take place in the form of more frequent hand washing, staying home when possible, and wearing protective gear when leaving the house. As a result, the spread of the disease can be decreased.
U.S.
Developing Apple and Google Contact Tracing App
Singapore
TraceTogether App
South Korea
Use of surveillance cameras
Credit card histories at restaurants