The high incidence of infectious diseases in neonatal broilers is largely due to immune system immaturity, marked by reduced cytokine production (IL-1β, IL-4, IL-10, IFN-γ), low IL-2 expression in the spleen, and decreased IL-4, IFN-γ, and Lysozyme C expression in the ileum between days 6 and 136-8,9. This evidence concerns the gene IL4 and infectious disease.