In this SIR model, the probability q of infecting susceptible neighbours reflects the characteristic infection rate of the particular disease over the simulation time step Dt. For a given time step size Dt, a more infectious disease will have a larger q, whereas a less infectious disease will have a smaller q. For a given infectious disease, q will be smaller for a smaller Dt, and larger for a larger Dt. The simulation time step Dt itself is chosen, for simplicity, to be roughly equal to the typical incubation and recovery period of the disease (about 3-5 days for common cold). This implies that t R <Dt, i.e. the infected nodes recover after one simulation time step.