130 BACTERIOLOGY. 
causing infection. As complete a knowledge of these 
facts as possible is necessary if we are to combat the 
spread of infection. In the superficial layers of the 
epithelium and on the surface of the skin we find the 
different pyogenic cocci, which are capable of infecting 
a wounded or injured part or causing inflammation in 
the glands. Acne, the pustules in smallpox, the pus 
on a burned surface, boils, ete., all come from the 
pyogenic cocci. In surgical cases the skin has to be 
as thoroughly disinfected as possible, to prevent the 
formation of stitch-hole abscesses and wound-suppura- 
tion. 
In the secretion of the mucous membrane covering 
the pharynx and nasopharynx there is always an abund- 
ance of bacteria. In one hundred throats examined by 
the writer in New York City, streptococci and staphylo- 
cocci could be found in over 90 per cent., and pneumo- 
cocci were very frequently discovered. Many other 
varieties of bacteria, such as the influenza bacilli, are 
probably often present in small numbers. In those con- 
stantly in contact with cases of diphtheria, and in those 
convalescent from diphtheria, virulent diphtheria bacilli 
are frequently found in the throat. 
After exposure to cold or injury of any kind, owing 
to the presence of these bacteria, the persons harboring 
them may develop tonsillitis, tonsillar abscess, or diph- 
theria; or the bacteria may invade the bronchial mucous 
membrane or the lungs. The diphtheria bacilli, and 
_ perhaps other bacteria, transmitted to others may be- 
come the source of infection to them, though the person 
who spreads the infection may remain unaffected. 
The stomach, on account of the acidity of its con- 
tents, is comparatively free from bacteria. The normal 
