I'm curious about your point/intentions with the bolded language in your post. Would you not recommend someone close their mouth in the shower because doing so probably matters only a few out of hundreds of showers? One should take a cumulative approach that fits best for them - each thing someone does can incrementally improve their chances to avoid sickness...no one thing covers everything that may cause sickness. Others covered many things before I posted. I simply added another potential precaution that one can choose (or choose not) to look into for themselves.