SIR model on a Barabási-Albert network. Hubs spread disease far faster than leaves.
Barabási-Albert networks grow by preferential attachment: new nodes link to existing nodes with probability ∝ degree. This creates a power-law degree distribution P(k) ∝ k⁻³. Scale-free networks have p_c → 0: any β > 0 gives epidemic. Hub infection (large blue node) spreads orders of magnitude faster than leaf infection.