Why not study the Human Immune System and how to reinforce it via diet. Like how they studied bone loss decades ago and decided to add Vitamin D to milk to incr

Why not study the Human Immune System and how to reinforce it via diet. Like how they studied bone loss decades ago and decided to add Vitamin D to milk to increase calcium uptake. Seems to me there are millions of potential pathogens that could be problematic for humans, increasing defenses seems to be the rational approach, if one’s intent is to actually preserve human life.