The spreading of an epidemic is determined by the connectivity patterns which underlie the population. While it has been noted that a virus spreads more easily on a network in which global distances are small, it remains a great challenge to find approaches that unravel the precise role of local interconnectedness. Such topological properties enter very naturally in the framework of our two-time-step description, also providing an approach to track a probabilistic system. The method is elaborated for SIS-type epidemic processes, leading to a quantitative interpretation of the role of loops up to length 4 in the onset of an epidemic.

• Received 18 December 2003

DOI:https://doi.org/10.1103/PhysRevE.69.066116