“…This means that the mobility in each subclass is influenced by the spatial gradient of others subclasses (see e.g. [8,9,21]). A prototype of a nonlinear system that governs this S-I-R model with cross-diffusion is, (d 1 + k 1 u 1 + δ 1,2 u 2 + δ 1,3 u 3 )∇u 1 + δ 1,2 u 1 ∇u 2 + δ 1,3 u 1 ∇u 3 · η = 0, δ 2,1 u 2 ∇u 1 + (d 2 + δ 2,1 u 1 + k 2 u 2 + δ 2,3 u 3 )∇u 2 + δ 2,3 u 2 ∇u 3 · η = 0,…”