In contact tracing, health workers identify and reach out to people who have an infectious disease, then try to determine with whom they have been in close contact and to whom they may have transmitted the disease. Then they try to reach those potentially exposed people to try to halt further infections. Dr. Darpun Sachdev, the contact tracing program lead in San Francisco, and Luis Hernandez, a local case investigator, talk about how contact tracing works, who is doing the job and what the latest contact tracing data reflect.