Whilst the greatest effort has been made to ensure the quality of this text, due to the historical nature of this content, in some rare cases there may be minor issues with legibility. The popular mind is prone to inquire about the existence of certain things or entities, rather than their quantitative relations; it asks, is there infection in this disease or in that? It does not think to inquire whether there is more or less infecting power; it does not suspect that this is the only difference in many diseases with regard to their power of propagating themselves.