INFLUENZA has always been a mixture of romance and terror; of fact and fable; of new and old ideas. Its various popular names: the jolly rant, the new delight, the newe acquayantance, gallants' disease, the fashionable illness, influenza di freddo or influenza di, coeli, la grippe, flu, this virus thing-all indicate a light-hearted annoyance. But interspersed are the tales of damaging experiences. The sweating sickness of England of 1485 which decimated Richmond's Army after the defeat of Richard III at Bosworth Field is thought by some to have been influenza. The pandemic of 1743 appears to have been calamitous. The term, blue plague, accredited to Horace Walpole, or the name Blitzkatarrh forecast features of the devastating episode of 1918. The historical disease, nevertheless, conforms in its epidemiological and clinical characteristics to what we now call influenza. That disease in its sporadic, epidemic, or pandemic form is caused by one of the influenza viruses. The influenza viruses comprise four types: A, B, C, D. The Type A viruses with which we shall here be concerned, comprise, in turn, four groups or families whose members or strains may vary in some characteristics, but retain a close group relationship (fig. 1).(The numbering of these families is inconsistent, because it is difficult at present to decide what order to assign to them.) Influenza virus is considered to consist of a core of RNA which is much the same in all strains