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of the creek for two or three weeks must have been quite heavily 
charged with typhoid bacilli, so that probably the majority of the 
milk cans washed in this water received some of the organisms. 
It is easy to understand how a milk supply thus almost if not quite 
continuously infected for several weeks may cause the infection of a 
large proportion of the persons who use that milk; but when the 
infection is introduced into the milk at irregular intervals for a like 
period, as would be expected when the infection is conveyed on the 
hands or clothing of persons or by flies, etc., a very small proportion 
of the consumers of the milk may become infected. 
The susceptibility of the people supplied with infected milk, of 
course, would affect the proportion. In a community where typhoid 
fever had been prevalent for years, and in which there would be a 
number of persons rendered relatively immune by previous infection, 
we would expect less susceptibility than in a community where the 
disease had never prevailed. 
That it takes susceptibility plus exposure to infection for the 
disease to occur was strongly suggested by an instance in the course 
of a milk outbreak in the District of Columbia in the fall of 1906. In 
a children’s home having about 100 inmates 7 children came down 
with typhoid fever within a period of two or three days. The way in 
which the milk was delivered to and served at the institution made it 
practically impossible for the 7 children affected to have drunk milk 
from any one can, or one day’s delivery, from which at least 75 per 
cent of the children did not drink. Thus of 75 children almost cer- 
tainly drinking infected milk only 7 had the disease. It is conceiv- 
able that in such an instance the typhoid bacilli in the can or cans of 
infected milk either were very few in number or that they were not 
uniformly distributed through the bulk of the milk, so that only one 
or two of the children drinking from a 5-gallon can of milk actually 
received any of the bacilli; but it seems much more reasonable to 
conclude that all of the 75 children received some of the bacilli and 
the escape of the majority was due entirety to lack of susceptibility 
at the time the organisms were ingested. 
It seems quite probable that different strains of the typhoid bacillus 
vary markedly in their infectiveness. The writer has become im- 
pressed with this view by observing in the course of his studies of 
typhoid fever in the District of Columbia frequent instances in which 
there are one or more cases of typhoid fever in a household in a most 
unhygienic and crowded neighborhood, many persons having free 
association with the patients, the excreta of the patients being handled 
with the grossest carelessness, flies swarming over the excreta as well 
as over the food for the sick and well, and yet under these apparently 
very favorable conditions for the spread of typhoid-fever infection 
