THE IMMENSE CHALLENGE. “The total number of microbes (∼10(14)) normally inhabiting our mucosal surfaces exceeds by an order of magnitude the quantity of cells (∼10(23)) in our bodies. Scattered amidst these commensal microflora are potential pathogens—viruses, bacteria, or parasites, intrinsically capable of producing symptomatic infectious disease.”
http://www.sciencedirect.com/science/article/pii/S0092867406001917