In classic public health practice, human contact tracers interview the newly infected, attempting to jog their memory and find all the epidemiologically significant encounters that might have passed on infections. The tracers contact exposed people, and persuade them to self-isolate and get tested. The effort has to be local, since most of the contacts happen locally. When the infection is in its infancy, with hot spots, rapid contact tracing and isolation can be very effective, as proven in many countries. Rapid contact tracing needs technology support. Many countries have used intrusive contact tracing apps.