On these infection trees, two types of contact tracing are distinguished. The first type is single-step contact tracing, in which all traceable contactees of a symptomatic case are quarantined, but tracing only continues when an undiscovered infected, quarantined in this way, is detected itself ( Figure 1A ). The second type is iterative tracing, in which tracing of traceable contactees is continued, and a whole cluster of infecteds that is linked through traceable contacts is quarantined ( Figure 1B ). Such continuation would be possible if a test were available to determine the infection status of traced contactees.