How do you build up a child's immune system so he won't catch so many illnesses?

GOOD diet: I sound like a "broken record" but it must be obvious to all that the only way that you can use an engine is by obeying the fuel rules in the owner's manual. The human body must have the right fuel for its cellular engines. We are wondrously made and all that is reqired is energy to run those engines. That comes from studying the right diet. It might require some supplements.