  The critical behavior of the stochastic susceptible-infected-recovered model
on a square lattice is obtained by numerical simulations and finite-size
scaling. The order parameter as well as the distribution in the number of
recovered individuals is determined as a function of the infection rate for
several values of the system size. The analysis around criticality is obtained
by exploring the close relationship between the present model and standard
percolation theory. The quantity UP, equal to the ratio U between the second
moment and the squared first moment of the size distribution multiplied by the
order parameter P, is shown to have, for a square system, a universal value
1.0167(1) that is the same as for site and bond percolation, confirming further
that the SIR model is also in the percolation class.
