Infection and contact tracing, on the other hand, are contact processes, and so take place at a rate proportional to a number of partnerships in the population. The full set of partnership links can be thought of as forming a network, through which contact processes spread. For infection, the rate is t½S/I, where the term in brackets is the number of susceptible-infectious pairs in the population and t is the transmission rate constant, and for tracing, the rate is r½I/T, where the term in brackets is the number of infectious-traced pairs in the population and r is the tracing rate constant. We have introduced here a notation in which a arrow is drawn from a state that transmits across the link to the state that is affected by the transmission, which will become important when we consider triples in addition to pairs.