_Secondary invaders._—We have considered a possible manner in which the virulence of the organism causing influenza may become enhanced. Whatever this organism may be, another and equally important virulence enhancement occurs in the opportunist group of the germs, so-called secondary invaders of influenza. As we have previously remarked, it is a characteristic of influenza outbreaks in all communities that the earliest cases are very mild. Secondary infection has not as yet obtained a foothold. After about a week the character of the illness changes, becoming distinctly more severe. Billings reported that at Camp Custer in the autumn of 1918 cases admitted to the hospital during the first five days were very mild in character and were reported as simply bronchitis or pharyngitis of no great severity, the majority soon recovering. After this time the entire symptom complex seemed to change and the cases admitted to the hospital were of a very different and more severe type. Benjafield reports that in the Egyptian Expeditionary Force the epidemic commenced in May, 1918, and that the cases occurring during the earlier portion of the epidemic were mild in type and of short duration, only a very small proportion being complicated by bronchopneumonia. Wooley found at Camp Devens that the first cases were of a mild form and were usually diagnosed “naso-pharyngitis, acute catarrhal.” After a few days the disease became more severe and pneumonia cases developed. Bezançon found that among the repatriated French soldiers from Switzerland those cases occurring in May and June had a much lower severity than in the later epidemic. Zinsser’s description of the mild, earlier epidemic in Chaumont has already been quoted. The secondary invaders of pathogenic importance are the various forms of the streptococcus and pneumococcus, the meningococcus, the staphylococcus, and probably the tubercle bacillus and the influenza bacillus. In the last epidemic as in that of thirty years previously, the chief complications were bronchitis and pneumonia. Capps and Moody found these to be the chief complications in December, 1915. Also they found a high incidence of sinusitis. This has been a feature of the last epidemic. Wooley cites a good example of the damage done by these opportunist organisms when they are present. Among the troops stationed at Camp Devens in the fall of 1918 pneumonia following influenza was particularly prevalent in a battalion of negroes from the South. This battalion had, a short time previously, passed through an epidemic of pneumonia and Wooley believes that many of the blacks were harboring the pneumococci which were only awaiting a favorable opportunity to invade their hosts. The influenza furnished the required opportunity.