Threshold Conditions for Arbitrary Cascade Models on Arbitrary Networks

Given a network of who-contacts-whom or who links-to-whom, will a contagious virus/product/meme spread and ´take-over´ (cause an epidemic) or die-out quickly? What will change if nodes have partial, temporary or permanent immunity? The epidemic threshold is the minimum level of virulence to prevent a viral contagion from dying out quickly and determining it is a fundamental question in epidemiology and related areas. Most earlier work focuses either on special types of graphs or on specific epidemiological/cascade models. We are the first to show the G2-threshold (twice generalized) theorem, which nicely de-couples the effect of the topology and the virus model. Our result unifies and includes as special case older results and shows that the threshold depends on the first eigenvalue of the connectivity matrix, (a) for any graph and (b) for all propagation models in standard literature (more than 25, including H.I.V.) [20], [12]. Our discovery has broad implications for the vulnerability of real, complex networks, and numerous applications, including viral marketing, blog dynamics, influence propagation, easy answers to ´what-if´ questions, and simplified design and evaluation of immunization policies. We also demonstrate our result using extensive simulations on one of the biggest available social contact graphs containing more than 31 million interactions among more than 1 million people representing the city of Portland, Oregon, USA.