═══════════════════════╤═══════════════════════╤═══════════════════════ │ Per cent. of exposed │Per cent. of those who Type of exposure in │individuals who had had│had not had a previous 1920. │ influenza in 1918–19 │ attack, and who on │ and who contracted it │ exposure contracted │ again, per cent. │ influenza, per cent. ───────────────────────┼───────────────────────┼─────────────────────── “Sleeping” │ 27.0│ 31.0 “Room” │ 18.3│ 17.6 “Family” │ 12.0│ 11.2 ───────────────────────┴───────────────────────┴─────────────────────── _On the whole there is no evidence of protection afforded by a previous attack._—Individuals who had had the disease before succumbed to a second attack in the same proportion as those who had not previously had influenza. _Recurrent cases._—In certain families there were individuals who had had influenza during both the 1918 and 1920 epidemics. Were these recurrent cases the first ones to occur in the family, or did they, as a rule, follow other cases in the same household? We have records of 236 recurrent cases in which we know the order of occurrence of the various cases in the family. Out of this total number 57 were the initial cases in the household. One hundred and nineteen were the only cases occurring in the family. Therefore 176 or 74 per cent. of the total number of recurrent cases were either the first or the only cases in the family. Sixteen recurrent cases followed between other cases and 44 occurred as the last of a series of two or more in the household. SECTION V. IMMUNITY. Opinions of all observers who have studied in detail the question of immunity in influenza are remarkedly in accord. The conclusions reached by Parkes in 1876 are valid today, and form as excellent an abstract of our present knowledge as any produced since his time. “There is some discrepancy of evidence, but, on the whole, it seems clear that, while persons seldom have a second attack in the same epidemic (though even this may occur), an attack in one does not protect against a subsequent epidemic. Indeed, it has been supposed rather to render the body more liable.” In 1890, Abbott wrote: “There is but little if any evidence in support of the protective power of one attack to confer immunity against a second; and hence adults are not exempt, as they usually are in epidemics of scarlet fever or other exanthemata; so that the proportion of adults to children attacked in an epidemic is necessarily greater than that which is observed in epidemics of other infectious diseases.”