Dr Birx stated about a month ago that the severity of the symptoms are a function of (1) your general health condition but (2) the 'viral load' that initially infected you. Brief exposure to covid 19 of a healthy individual, few or no symptoms even though you might test positive. Long-term, up close exposure (hospital workers around a sea of positive patients) more severe symptoms. So, your comment about developing constant immunity due to regular exposure in the general community makes sense to me.