92 SAPROPHYTISM, PARASITISM AND PATHOGENISM 
suspended in the air for some time. The possibility of droplet infec- 
tion has been definitely proven in the following manner: Agar plates 
containing sodium carbonate are placed at various heights and distances 
from the experimenter, who places in his mouth a solution of phenol- 
phthalein and then talks in a natural manner, expelling droplets 
containing phenolphthalein during his speech. This dye is transmitted 
with the droplets until they reach the agar plates, where bright red 
spots are produced which are very readily observed. In like manner, 
cultures of B. prodigiosus placed in the mouth will infect agar plates 
at similar distances. 
The transmission of disease by droplet infection may be, and fre- 
quently is, a very direct one. Teague^ detected diphtheria bacilli 
in the invisible droplets emitted by over one-half the patients when 
they coughed or sneezed, and Alice Hamilton- and Weaver^ found 
hemolytic streptococci in the droplets emitted by a considerable propor- 
tion of scarlet fever patients who cough. Bacteria which are air-borne 
or borne by droplets may remain alive for several weeks in indirect 
sunlight, but all of them are killed readily if they are exposed to direct 
sunlight. Kirstein^ cultivated diphtheria bacilli forty-eight hours after 
they were emitted from the mouth, and Staphylococcus aureus after 
a w^eek. The virus of whooping cough, mumps, measles, influenza, 
cerebrospinal meningitis, pneumonic plague, tuberculosis, the exan- 
themata, 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 over- 
crowding occurs, as in tenements, public gatherings, railway trains, 
schools and factories. Weaver^ has studied the important question 
of the prevention of droplet infection by the use of the face mask, a 
very important problem in contagious hospitals, and finds that they 
appear to prevent the spread of infection by the patient, and protect 
those who are attendant upon those sick persons who emit droplets. 
It is important to shield the eyes, also. Certain bacteria have been 
found to pass from the conjunctiva through the lachrymal duct to the 
nose. This precaution is particularly desirable during epidemics of 
pneumonic plague and "flu." 
2. Soil-borne Infection.— 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. Of those bacteria which are 
habitually pathogenic for man, typhoid, cholera, paratyphoid and 
' Jour. Infec. Dis., 1913, 12, 398. 2 jour. Am. Med. Assn., 1905, 44, 1108. 
5 Jour. Infec. Dis., 1919, 24, 218. * Ztschr. f. Hyg., 1902, 39, 93. 
* Loc. cit. 
