The number of epidemics is equal to the number of seeds. Initially the only nodes that are infected are the seeds. Once a node is infected by some epidemic it cannot be infected by another, thus the epidemics compete over the nodes they can infect. In each iteration of the algorithm, one node is infected. The probability of infection by a given epidemic is linearly proportional to the number of neighbors that are infected by that epidemic.

Ali Salehi 2005-04-29