PARASITIC AND PATHOGENIC BACTERIA . 93 



but all of them are readily killed if they are exposed to direct sunlight. 

 The virus of whooping-cough, mumps, measles, influenza, cerebro- 

 spinal meningitis, pneumonic plague, tuberculosis, the exanthemata, 

 the diphtheria bacillus, and possibly the pneumococcus may be spread 

 in this manner. Air-borne infections probably rarely take place 

 in the open air where the sunlight is strong. This does not apply 

 to droplet infections where one individual coughs, talks or sneezes 

 directly into the face of another. Air-borne infections, particularly 

 droplet infections, are potentially common where overcrowding occurs, 

 as in tenements, public gatherings, railway trains, schools, and factories. 



2. Soil-borne Infections. Those bacteria which are occasionally 

 pathogenic for man and produce sporadic disease in man, and whose 

 habitat is the soil, are for the most part spore-forming organisms. They 

 commonly enter the body through wounds. Of these the bacillus of 

 tetanus, malignant edema, symptomatic anthrax, of anthrax, and the 

 gas bacillus are the best known but, with the exception of the latter, 

 they are not habitually human parasites. Of those bacteria which 

 are habitually pathogenic for man, typhoid, cholera, paratyphoid 

 and probably dysentery may be soil-borne, but ordinarily infection 

 with these organisms does not take place through the soil. 



3. Water-borne Infection. The viruses of excrementitious diseases 

 typhoid, paratyphoid, dysentery, and cholera are not infrequently 

 transmitted from man to man through contaminated water. Feces 

 containing these organisms get into water supplies, reach man again, 

 incite disease in man, again escape in the feces and reenter water 

 courses, thus being recirculated. The cycle may be somewhat more 

 complex, as for example, when typhoid dejecta are thrown upon the 

 ground and are eventually washed directly into water supplies and 

 thus reach man again. 



4. Food-borne Infection. A considerable number of pathogenic 

 bacteria may reach man through food, although food which is infected 

 is usually rendered so through the handling of it by man. Milk is 

 probably the most common food thus to be infected and it is par- 

 ticularly dangerous for two reasons. In the first place, its opacity 

 makes it difficult to distinguish foreign substances which may be in it; 

 and again, it contains all the elements which are necessary for the 

 food of man and incidentally for the majority of bacteria. Scarlet 

 fever, diphtheria, tuberculosis both human and bovine, Malta fever, 

 epidemic sore throat or tonsillitis, typhoid, dysentery, foot-and-mouth 

 disease, many diarrheas of children, milk sickness, and the organisms 



