io THE POPULAR SCIENCE MONTHLY 



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. Prom 

 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 infec- 

 tion ; 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 atmos- 

 phere. 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 



