Wash hands; use mask. For a person with a weak immune system, he should get the vaccines he is permitted to take (usually any non-live vaccine). He should wear a mask when around other people, especially anybody with sniffles or cough (including people who claim to have "just allergies"). Avoid shaking hands if possible. Otherwise, use hand sanitizer, and wash hands after touching possibly "germy" surfaces.
Immune System. Getting your yearly vaccines is a good start, good diet and exercise, daily multivitamin, treating your medical illnesses appropriately. If you have a documented immune deficiency, then working with your immunologist is a must.