METHODS AND CHANNELS OF INFECTION. 525 



of material quickly settle down, but the small, almost invisible pieces of 

 dried material may remain suspended for three or four hours. It is these 

 small particles which are usually inhaled or deposited on the skin and 

 mucous membranes of normal individuals that produce infections. 

 Bad. tuberculosis may be carried in this way as well as many other 

 pathogens. The extent of such dissemination is not well established. 



Droplet Infection. It has been demonstrated that during the process 

 of talking, coughing and sneezing, small bubbles or droplets of sputum 

 are thrown out into the air. These particles remain suspended for some 

 time and may be inhaled or deposited elsewhere. It is surprising the 

 distance that these small particles may be carried. It is stated that 

 they may be thrown out twelve or more meters. It has been shown 

 that Bact. tuberculosis is rarely thrown out over one and a half meters 

 by the cough of the tuberculous individual. It should be remembered 

 that these bacteria will remain alive two to three weeks when in the 

 dark but that they live only a few hours when exposed to the light. Of 

 the pathogenic microorganisms and viruses which are disseminated 

 by droplets of moisture, may be mentioned those of whooping cough, 

 mumps, measles, influenza, epidemic meningitis and pneumonia. 



Air-borne infections rarely occur and are not of great importance 

 in the open air where light has free access, but this type of infection 

 occurs in crowded quarters such as dark shops, schools, tenements and 

 railway trains. 



WATER-BORNE . INFECTIONS. Pure infections of this type occur in 

 practically only four diseases, namely Asiatic cholera, typhoid fever, and 

 in dysentery of the amoebic and bacillary forms. The drinking of water 

 or of fluids or materials contaminated by water is only one of the ways 

 in which these diseases are acquired. 



INFECTIONS FROM SOIL. The soil as a source of infectious micro- 

 organisms is of prime importance in only a few diseases, namely, anthrax, 

 tetanus, symptomatic anthrax, malignant oedema, emphysematous 

 gangrene, Asiatic cholera, and typhoid fever. In the first five mentioned 

 infection always takes place through some wound usually in the skin 

 and in the last two diseases mentioned infection is usually through the 

 intestinal tract but may also occur by means of wounds. The micro- 

 organisms of anthrax, tetanus and emphysematous gangrene, or more 

 specifically the spores, will remain in soil for long periods of time. They 

 are sometimes found in the active vegetative stage but it is probable that 



