  By means of numerical simulations and epidemic analysis, the transition point
of the stochastic, asynchronous Susceptible-Infected-Recovered (SIR) model on a
square lattice is found to be c_0=0.1765005(10), where c is the probability a
chosen infected site spontaneously recovers rather than tries to infect one
neighbor. This point corresponds to an infection/recovery rate of lambda_c =
(1-c_0)/c_0 = 4.66571(3) and a net transmissibility of (1-c_0)/(1 + 3 c_0) =
0.538410(2), which falls between the rigorous bounds of the site and bond
thresholds. The critical behavior of the model is consistent with the 2-d
percolation universality class, but local growth probabilities differ from
those of dynamic percolation cluster growth, as is demonstrated explicitly.
