CHAPTER VII. 
INFECTION. 
THE spread of infection is influenced by: 1. The 
number of species of animals subject to infection. 
Many human infectious diseases do not occur in ani- 
mals, and many animal infections are not found in man. 
Thus, so far as we know, gonorrhcea, syphilis, measles, 
smallpox, typhoid fever, ete., do not occur in animals 
under ordinary conditions; while tuberculosis, anthrax, 
glanders, hydrophobia, and some other diseases are 
common to both man and animals. 
2. The quantity of the infectious material thrown off 
from the body and the prevalence of the disease. 
In diphtheria, typhoid fever, cholera, pulmonary 
tuberculosis, septic endometritis, influenza, and gonor- 
rhea enormous numbers of infectious bacteria are cast 
off through the discharges from the mouth, intestines, 
and genito-urinary secretions, causing great danger of 
infection. On the other hand, in tubercular perito- 
nitis, cerebro-spinal meningitis, septic endocarditis, 
gonorrheal rheumatism and the like, there is little or 
no danger of infecting others, as few or no bacteria are 
cast off. 
3. The resistance of the infectious bacteria to the 
deleterious effects of drying, light, heat, ete. 
In this case the presence or absence of spores is of 
the greatest importance. The spore-bearing bacilli, 
