  Infectious diseases are caused by pathogenic microorganisms, such as
bacteria, viruses, parasites or fungi, which can be spread, directly or
indirectly, from one person to another. Infectious diseases pose a serious
threat to human health, especially COVID-19 that has became a serious worldwide
health concern since the end of 2019. Contact tracing is the process of
identifying, assessing, and managing people who have been exposed to a disease
to prevent its onward transmission. Contact tracing can help us better
understand the transmission link of the virus, whereby better interrupting its
transmission. Given the worldwide pandemic of COVID-19, contact tracing has
become one of the most critical measures to effectively curb the spread of the
virus. This paper presents a comprehensive survey on contact tracing, with a
detailed coverage of the recent advancements the models, digital technologies,
protocols and issues involved in contact tracing. The current challenges as
well as future directions of contact tracing technologies are also presented.
