For determining the cost-effectiveness of contact tracing, not only the critical tracing probability p c * is required, but also how easily that p c * can be achieved. A key aspect will be the possibility to distinguish relevant contacts: for control of sexually transmitted diseases relevant contacts are easily identified (that is, if people are willing to co-operate), but in case of respiratory pathogens like SARS or influenza the notion of relevant contacts is rather diffuse. Good insight will be obtained by not only regarding the sensitivity of tracing (p c ), but also the positive predictive value: what proportion of traced individuals is actually infected, and if we put more effort into increasing p c , how will it decrease the positive predictive value?