SIR EPIDEMIC — Scale-Free Network (Barabási-Albert)

0.15
0.10
150
R₀ = β/γ·⟨k²⟩/⟨k⟩ = —
Scale-free networks (BA model, P(k)~k−3) have threshold β_c → 0 as N→∞ — epidemics spread at ANY rate! Contrast with Erdős-Rényi: finite threshold β_c = ⟨k⟩/(⟨k²⟩−⟨k⟩). Hubs = superspreaders. Targeted hub immunization far more effective than random vaccination.