Illustration of critical slowing down in the potential function of the linearized SIR model. Disease prevalence (I/N) is represented by the horizontal position of a ball sliding through viscous fluid in a bowl with height determined by the potential function. Both the depth of the bowl and the viscosity of the fluid in the equivalent physical system are affected by vaccine coverage ν. The bowl is shallowest near the immunization threshold, which illustrates the slowing down of the dynamics as the critical point (ν ≈ 0.941, green dashed line) is approached. Oscillatory dynamics occur above immunization level ν ≈ 0.939, corresponding to the system becoming underdamped (pink region). Original figure online at https://doi.org/10.1371/journal.pcbi.1006917.g002