Abstract: Methods and software for tracing contacts among people via exchanging of contact-tracing (CT) keys between mobile devices and treating various ones of the CT keys as infected CT keys after self-reporting events in which users have self-reported as having, at least potentially, been infected by a target infectious agent. In some embodiments, CT keys are exchanged between mobile devices when the mobile devices determine that they arm within a predetermined distance of one another and/or for a predetermined amount of time such that sufficient contact for infectious-agent transmission could have occurred. In some embodiments, mobile devices within the predetermined distance of one another can determine whether or not they are within a same separated space as one another (e.g., are not separated by a physical partition) as an additional level of intelligence to inform whether or not transmission of the infectious agent could have occurred.