A paradigm for absorbing-state phase transitions: sites infect neighbors at rate λ, recover at rate 1. Critical λ_c ≈ 1.6488 in 1D.
The contact process is the simplest model in the directed percolation (DP) universality class. Below λ_c the infection dies; above it a steady active phase persists. DP critical exponents: β ≈ 0.2765, ν_⊥ ≈ 1.097, ν_∥ ≈ 1.734.