120 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
using the most milk, generally women and children. 3. 
The period of incubation is relatively short. 4. The dis- 
ease is of mild type. 5. The mortality is lower than usual. 
As a rule, the conclusion that the disease is dissemi- 
nated by milk must be based upon these characteristics 
and upon information obtained regarding the manner in 
which the milk may have been infected, rather than upon 
the demonstration of the infectious agent in the milk. 
The cause of scarlet fever is not known and consequently 
its presence cannot be detected by any known method of 
examining milk. The bacillus of typhoid fever has been 
demonstrated in milk several times, thus affording posi- 
tive proof that this organism is transmitted by milk, but 
the examination has been unsuccessful in a much larger 
number of cases. The diphtheria bacillus has been re- 
covered from milk in even fewer instances. There are 
several reasons why efforts to isolate these organisms 
from milk which is the cause of an epidemic may be unsuc- 
cessful. 1. Only a small quantity of the milk, a drop or 
two, is subjected to examination, and this may be free 
from the organisms even when the latter are relatively 
numerous in the whole volume of milk concerned. 2. The 
period during which the milk is infected may be termi- 
nated before it is suspected and examined. 3. The or- 
ganisms may be overgrown by the other kinds of bacteria 
which are present in milk in greater number. In prac- 
tice, the presence of these infectious agents in milk is 
not suspected until several cases of disease have appeared. 
Even if they could be detected in milk with more cer- 
tainty, it would be a mistake to defer action after an 
epidemic has started until the milk can be examined, 
because this would allow more time for the dissemination 
of the infection. 
