38 
persons exposed to the infection is small. Or it may be mixed in a 
dairy with that from many other cans, and thus a larger amount of 
more dilute infectious material be delivered to the community. If 
kept cool, the milk may remain thus dilute as regards the pathogenic 
organism and the disease may crop out among the consumers only in 
those most susceptible or in persons drinking a comparatively large 
amount. But if the milk becomes warm because of lack of care or 
long transit and the contained organism is such that it will prolif- 
erate in milk, each quart delivered to the consumer may be more 
infectious than the original can. In either case the users of the 
infectious milk will receive their dosage of the organism at approxi- 
mately the same time and will therefore, making due allowance for 
the normal variations in jDeriod of incubation, fall ill simultaneously. 
The initial explosion may therefore consist of but a few cases if 
the amount of infected milk is small or if very dilute: or of many 
if the amount of infected milk is large or the number of organisms 
great. If the milk is infected at but one milking, the outbreak will 
rapidly subside and. aside from secondary cases spread by contact 
or other means, no new ones will appear. If the milk is infected 
day after day. the outbreak continues: and in contagious diseases, 
after the lapse of the period of incubation from the initial outbreak, 
secondary cases due to contact are apt to appear and grow more 
numerous, so that the picture presented of a typical milk epidemic 
may become less clear. A milk epidemic is therefore most typical 
in its onset, although under efficient systems of notification and quar- 
antine secondary contact cases may be largely prevented and it may 
then maintain its characteristics throughout. The development of 
these secondary cases contracted by contact and otherwise explains 
why in the epidemics reported later in tabular form not all of the 
cases in the outbreak are reported as consumers of the suspected 
milk. Another explanation is that in most cities there are always 
a certain small number of cases of the commoner contagious diseases 
which have varied sources of origin and may be termed residual. 
It is on top of this as it were that an epidemic occurs. 
(b) Disease foil ores the milk. — Disease carried by milk can occur 
only among users of the infectious milk. Milk routes may therefore 
at times be considered thoroughfares of infection. During an epi- 
demic other cases of .course may occurr among nonconsumers, but 
the contagion is carried to these by other means. In typhoid fever 
for example it is possible to conceive a water and a milk epidemic 
occurring at the same time, or what is possibly more common in 
the cities, during an unusual prevalence of typhoid due to various 
and in some cases unknown causes, smaller milk outbreaks may occur. 0 
n Bull. Xo. 35, Hyg. Lab., U. S. Pub. Health and Mar.-Hosp. Serv., Wash., p. 20. 
