This question cannot be definitely answered, and yet it is one of extreme importance, particularly with regard to prevention and combat of the disease. How universally is the influenza virus distributed during pandemics? What proportion of the population is actually exposed by invasion with the virus? What proportion of actually exposed individuals develops the disease? We will refer to this again when comparing influenza with other infectious diseases, but it is of particular interest now to review our individuals who were exposed by sleeping with cases of influenza. Fifty-five per cent. of all individuals sleeping in the same bed with cases of influenza in 1918 _did not_ contract clinical influenza. Seventy per cent. of all individuals sleeping with influenza cases in 1920 _did not_ contract the disease, in recognizable form. Sixty-nine per cent. of all individuals in 1920 who had not had the disease previously and who slept with cases _did not_ develop evidences of the disease. It is difficult to conceive of a degree of exposure much closer than that of sleeping in the same bed with a sick individual. And yet it is equally conceivable that many individuals sleeping in the same bed with a patient were not penetrated by the virus of influenza. This does not aid us in answering our question. We do not know whether the more important factor is that of a natural immunity or that of absence of actual invasion by the virus. These results with sleeping contacts form an interesting link in the chain of evidence started during 1918 by the U. S. Navy and Public Health Service, and reported by Rosenau and by McCoy and others. These experimenters working in Boston and in San Francisco carried out inoculation experiments on human volunteers. The work in Boston, as reported by Rosenau, was carried on with 100 volunteers from the Navy between the ages of eighteen and thirty, most of them between eighteen and twenty-five; all of them entirely well, and with the exception of a few controls, none having experienced known attacks of influenza previously. First, suspensions of thirteen different strains of influenza bacilli, all from cases of influenza during the epidemic, were sprayed into the nose, eyes and throat of nineteen volunteers. None of them took sick. Next, secretions from the mouth, nose and throat and bronchi of acute cases of influenza were collected, pooled, and without filtration sprayed into each nostril, into the throat during inspiration, and onto the conjunctiva of each of ten volunteers. None of them took sick. Some of this same material was filtered through a porcelain filter and administered in the same manner, with similar results. One cubic centimeter of each type was administered to each individual. The interval between the time of collection and time of inoculation was then decreased to one hour and forty minutes, the minimum time in which the material could be transferred from hospital to experiment station.