The network-based disease transmission and contacttracing model is based on models considered by Huerta & Tsimring (2002) , Eames & Keeling (2003) and Kiss et al. (2005 Kiss et al. ( , 2006a . Each node in the network is classified according to one of four states of disease progression: susceptible nodes (S ); nodes at once infected and infectious (I ); nodes 'triggering' contact tracing (T ), which are identified as being infectious, are immediately isolated and initiate tracing; and, finally, removed nodes (R), which are no longer infectious and do not initiate tracing. The transitions between states are depicted in figure 1.