To investigate the spread of an infectious disease on a contact network, we use the following methodology: Individuals in a population are represented as nodes in a network, and the edges between the nodes represent the contacts along which an infection can spread. Contact networks are abstracted by undirected, unweighted graphs (i.e. all contacts are reciprocal, and all contacts transmit an infection with the same probability). Edges always link between two distinct nodes (i.e. no self loops), and there must be maximally one edge between any single pair of nodes (i.e no parallel edges). Each node can be in one of three possible states: (S)usceptible, (I)nfected, or (R)esistant/immune (as in standard SIR models). Initially, all nodes are susceptible.