Simple contagion

A simple contagion is a process in which a node in a network adopts a state or behavior after exposure to a single activated neighbor. Unlike complex contagion, which requires multiple exposures or social proof, simple contagion spreads through contact alone: one infected neighbor, one adopted belief, one shared exposure. The mathematics of simple contagion is the mathematics of epidemic models: the SIR model, the SIS model, and their network generalizations all describe simple contagion dynamics. The critical insight is that simple contagion spreads fastest through networks with short path lengths and many bridge edges — the topology of random networks and scale-free networks. Simple contagion is the default model for biological epidemics, computer viruses, and rumor spread, but it fails to capture social behaviors that require reinforcement or social proof.