how to do discetization of partial diffential equationn

∂S(r,t)/∂t= μ-β(t)IS-μS+D_1 ∇^2 S(r,t) (2a)

∂E(r,t)/∂t=β(t)IS-(μ+δ)E+D_2 ∇^2 E(r,t) (2b)

∂I(r,t)/∂t= δE-(γ+μ)I+D_3 ∇^2 I(r,t) (2c)

∂R(r,t)/∂t=γI-μR+D_4 ∇^2 R(r,t) (2d)

Here μ is the death rate per capacity, and 1/δ and 1/γ are
the mean latent and infectious periods, respectively, of the
disease. t is the rate of disease transmission between individuals.
The population can be normalized to S+E+I+R
=1, so all dependent variables represent fractions of the
population.

can i know how to discretize the PDE?beside that how to compute the pattern fromation using mathematica?this is the PDE for model cellular automata for dengue.thanks