CD4 and viral infectious disease: For example, interferon (IFN)‐γ from CD4 T‐cells protects against viral infections, while interleukin (IL)‐17 aids controls of bacterial and fungal infections, and IL‐4 protects against infection by parasitic worms.12, 13, 14, 15, 16, 17, 18, 19, 20 Most immune protection studies are carried out in animal models where challenge studies are feasible and mechanisms of protection can be identified by loss or gain of function.