is spread chiefly in its incipient stage, when the child has no other symptom than a slight running at the nose and goes to school or to a party, thinking it has only a cold in the head. The carrier, properly so-called, may be a convalescent, entirely cured so far as his own symptoms are concerned, but still distributing virulent germs. From three to five per cent, of all typhoid cases continue to discharge typhoid bacilli for three months or more after convalescence. A recent milk borne epidemic in New York was due to infection from a carrier who had had typhoid in the West forty-six years before. Malarial infection is very frequently introduced into a community by Italian laborers who have themselves become immune to the parasites they carry, in their own country years before. Finally we have the most striking type of carrier, in which there is no present or past history of active disease at all. The famous cook, Typhoid Mary, for example, caused epidemics in eight different families where she worked, but so far as is known had never suffered herself. Two or three persons in a hundred in any normal population have been found on bacteriological examination to be carriers of the diphtheria bacillus, two or three in a thousand, of the typhoid bacillus. The problem of the carrier is one of the most serious of those which confront us since he is moving about in the world, mixing with others, perhaps taking part in the preparation and handling of food, and so may be proportionately much more dangerous than the sick person who is confined to his bed and under medical care.
It is a great step forward, however, to have learned that the human body, of the sick person or the carrier, is the primary source of infection; that every case of communicable disease is caused by the transfer of infectious material from such a person to a susceptible victim; and that the extent of the danger varies directly with the immediacy of the transfer. As this conception has been worked out in detail it has become clear that many of the supposed dangers of the pre-scientific period were of altogether minor import. To the older sanitarian the atmosphere was full of vague dangers, but its part in the spread of disease is now known to be exceedingly limited. In the immediate vicinity of a sick person or a carrier the air is infected by fine spray thrown out of the mouth in coughing, sneezing or loud speaking; but this is really a form of contact, not a general infection of the atmosphere. The mouth spray is a sort of rain which falls quickly to the ground, where it dries and the disease germs perish. It is true that dust collected from the surface in streets and rooms contains vast numbers of harmless germs and occasionally some disease-producing types. I am myself of the opinion that there may be real danger in breathing in such gross quantities of dust as one sometimes encounters in a dirty street on a windy day. This again, however, like the mouth spray, is an occasional and local pollution. Bacteriological studies, such as those carried on last winter in the New York schools, show that
