This seems to run counter to the current hygiene hypothesis, which is the current mainstream scientific opinion on one aspect of allergy development. Children in developed countries are decreasingly exposed to various microorganisms, and this is thought to result in increased rates of allergic disorders.
So I think it's possible that precautions reduce viral and bacterial exposure, but since they don't completely stop it, immune systems may still get enough exposure to build immunity to things while still not getting enough exposure to actually make them sick.