Chris DThis is a great thread. I think no matter where we are on the "paranoia" spectrum, illness is something we all have to think about and deal with.
I am in the germaphobe category. (Although I have to admit I was there before my son was born!) I have heard the rationale about exposing kids to germs lets them build up their immunity and I believe it to a point. However, I think we also have to realize that our human environment has changed considerably in the last 100 years. It was not until recently, at least in the western cultures, that large groups of young children were thrown together (read public school systems). Eight siblings growing up on a farm with Mom and Dad simply were not exposed to 28 six year olds in a one room environment.
All that said, I do try to strike some balance. I am vigilant about wiping surfaces and washing hands wherever and whenever possible, but I try to not ever let it interfere with living our daily lives. There are some precautions I took when my son was smaller (he is now 2 1/2) that I am sure saved us some illnesses. For example, you can buy these Table Topper things that are like a sheet of plastic with adhesive on the back. They stick to resteraunt tables and provide a clean eating surface for infants and young toddlers. I worked in resteraunts for years and you would not believe how disgusting some of those tables are.
As others said, it also depends on your child's history of reacting to illness. I think it is perfectly normal for someone whose child has been repeatedly hospitalized to be a little paranoid. It is natural to not want to see your child hurting. At least for me, it has become easier as my son has aged. When he was an infant it was so difficult to tell whether he was coming down with something, or just teething etc. Then when he hit the stage of "everything in the mouth" it seemed an impossible mission to keep him healthy. I am sure when he hits school age and is more out of my control I will face other hurdles.
One last thought to my rambling post - I too think diet is critical. A while back someone posted good foods for the immune system. I hope they post again. I wrote them down at the time and was better about it last winter but I still remember some of the best foods were the orange fruits and veggies e.g. cantalope, pumpkin, squash.