The most striking clinical features of influenza are its epidemic character, its involvement of the respiratory tract, its extremely prostrating effect, and the often surprising rapidity with which the individual cures himself. These features strongly suggest that the etiologic agent of the disease is an organism subject to rapid changes in virulence; that it is confined to the respiratory tract where it produces a superficial inflammatory reaction giving rise to the characteristic symptoms of coryza, pharyngitis and tracheitis; that it elaborates a poison, possibly a true toxin, readily absorbed by the lymphatics, the effect of which is manifested in the profound prostration, severe aching pains, erythema, and leucopenia; and that it may either disappear promptly from the respiratory mucous membrane at time of recovery or may persist, leading a relatively saprophytic existence for an indefinite period of time, being no longer harmful to the individual, at least more than locally, because of an acquired immunity. Furthermore, in our opinion, the very brief incubation period suggests that the disease is bacterial in origin, rather than that it is analogous to the exanthemata, the majority of which present a comparatively long, fairly constant, incubation period. B. influenzæ has characteristics in accord with the clinical features of influenza. It is an organism of very labile virulence; it is always present in our experience on the mucous membranes of the respiratory tract in early uncomplicated cases of influenza, often in overwhelming numbers; in only very exceptional instances, in adults at least, does it invade the body producing a general infection, as the numerous reports of negative blood cultures testify; recent investigations by Parker[48] and others indicate that it is capable of producing a toxin quickly fatal for rabbits; it is predominantly present in the respiratory tract during the active stage of the disease and disappears in a considerable proportion of cases at time of recovery, while in others, more particularly those that develop an extensive secondary bronchitis and bronchiectasis it may persist for an indefinite period of time. It is, of course, fully appreciated that the foregoing is in the main merely argumentative reasoning and it is put forth only to suggest that B. influenzæ merits a much closer scrutiny with respect to its etiologic relationship to influenza than the trend of present opinion has awarded it.