It is impossible at the present time to decide whether the world epidemic spread simultaneously from these three foci or whether in only one of these three the virus became so exalted as to produce pandemic prevalence. All we can say is that we are able to trace consecutively the spread of the influenza from the focus in the United States throughout the world. The information upon which we base our findings is not statistical, and as we have previously said this latter type of demographic study should be brought into use to either corroborate or disprove our findings. VIRULENCE ENHANCEMENT. Before attempting to study the mechanism of origin of the 1918 pandemic it is highly essential that we devote some attention to a consideration of the processes by which the germs of infection, particularly the virus of influenza, may develop an increase in virulence. Followers of the theory of periodicity would base virulence enhancement primarily on some intrinsic property of the virus itself. We know from past experience and particularly from animal experiments that this is not the only manner by which virulence may be increased. As far as we know there is no new infectious disease. Individuals who delve into the history of the past inform us of more and more diseases which were well known to the ancients. We are frequently amazed at the variety of diseases now known to be infectious that were very correctly described by the Hippocratic writers. The infectious diseases are with us always and live nearly always in man, the host. There are few exceptions. Very few of the contagious viruses can live for any long period of time outside of the human body. A few, such as the plague bacillus, may live on other hosts, but these are the exception. The remarkable feature is that for long periods of time the virus exists in the host in a quiescent state and only at intervals does it become highly invasive and thereby produces epidemics of greater or less extent. Under what conditions does the metamorphosis of the microorganisms occur?