of time in the same room as the index case are relevant. Understanding the pathology and transmissibility of the disease guides the approach to contact tracing strategy. Outbreaks Although contact tracing is most commonly used for control of diseases, it is also a critical tool for investigating new diseases or unusual outbreaks. For example, as was the case with SARS, contact tracing can be used to determine if probable cases are linked to known cases of the disease, and to determine if secondary transmission is taking place in a particular community. Contact tracing has also been initiated among flight passengers during the containment phase of larger pandemics, such as the 2009 pandemic H1NI influenza. Contact tracing played a major role in investigating Ebola virus in the UK in 2014 and monkeypox in the UK in 2018. The eradication of smallpox, for example, was achieved by exhaustive contact tracing to find all infected persons. This was followed by isolation of infected individuals and immunization of the surrounding community and contacts at-risk of contracting smallpox. Contact tracing can help identify the etiology of a disease outbreak. In 1984, contact tracing provided the first direct evidence that AIDS may be spread by an infectious agent during sexual contacts. The infective agent has since been identified as HIV. Diseases for which contact tracing is commonly performed include tuberculosis, vaccine-preventable infections like measles, sexually transmitted infections (including HIV), blood-borne infections, Ebola, bacterial infections, and novel virus infections (e.g., SARS-CoV, H1N1, and SARS-CoV-2). Contact tracing has been a pillar of communicable disease control in public health for decades. With each outbreak and disease presenting with its own challenges, contact tracing is an adaptable tool used by authorities to identify, notify, and curb transmission of infections. Backward and forward tracing Backward (or reverse) tracing seeks to establish the source of an infection, by looking for contacts before infection. It can be used when a source of the infection is unknown and in situations of minimally monitored settings. The goal is identify the exposures associated with the index case and secondary exposures of those contact to halt the cycle of transmission. During the COVID-19 pandemic, the adoption by Japan in early spring 2020 of an approach focusing on backward contact tracing was hailed as successful, and other countries which managed to keep the epidemic under control, such as South Korea and Uruguay, are said to have also used