Contact tracing of symptomatic infecteds works on an intermediate level: treatment or quarantine of contactees (by contact we mean the possible transmission event, by contactee the individual that is contacted) may be effective because unidentified infecteds are most likely to be found among contactees, and efficient because the resources can be directed towards individuals at risk only. Tracing and quarantine has been successfully applied for smallpox control, where the term 'Leicester method' refers to exactly this strategy, with the establishment of specific smallpox hospitals [1] . It was also successful during the more recent SARS epidemic [2] , but not during the British foot-and-mouth epidemic [3] . Here we wish to investigate how these differences can be accounted for by the tracing process, thereby distinguishing tracing and quarantine as having separate effects which can be explained as follows.