If you don't get gastroenteritis for years does the immune system become less efficient at fighting it and thus causing severe symptoms when infected?

Consider this...: Lots of things can cause GI distress to recur--incompletely treated infection (e.g. Giardia) or re-exposure with re-infection; maldigestion (pancreatic insufficiency) or malabsorption (celiac sprue); food intolerances with re-exposure to the offending substance (lactose, sorbitol, etc.), irritable bowel syndrome, missed diagnosis (incomplete evaluation), non-gi related problems (uti, pid, etc.).