The only stupid question is the one that is not asked! There are no stupid or elemental questions. Look around the site, you may find answers, if not, ask away! Lots of people will jump in, not just me.
Your immune system question is a very good one, and one with no easy answer. Yes, it is evolving but also there are other important factors on how our organism interacts with new bugs. The bugs evolve, too! There are lots of unanswered questions there, lots of interesting research.