now PARASITIC AND PATHOGENIC BACTERIA REACH MAN 9o 
probably dysentery may be soil-borne, but ordinarily infection with 
these organisms does not take place through the soil. 
3. Water-borne Infection.— The viruses of excrementitioiis diseases- 
typhoid, parat\'phoid, dysentery and cholera— are not infrequently 
transmitted from man to man through contaminated water. Feces 
containing these organisms get into water supplies, reach man again, 
incite disease in man, again escape in the feces and reenter water 
courses, thus being recirculated. The cycle may be somewhat more 
complex, as for example, when typhoid dejecta are thrown upon the 
ground and are eventually washed directly into water supplies and 
thus reach man again. 
4. Food-borne Infection.— A considerable number of pathogenic bac- 
teria may reach man through food, although food which is infected 
is usually rendered so through the handling of it by man. Milk is 
probably the most common food thus to be infected and it is par- 
ticularly dangerous for two reasons. In the first place, its opacity 
makes it difficult to distinguish foreign substances which may be in 
it, and again, it contains all the elements which are necessary for 
the food of man and incidentally for the majority of bacteria. The 
organisms causing scarlet fever, diphtheria, tuberculosis, both human 
and bovine, Malta fever, epidemic sore throat or tonsillitis, typhoid, 
dysentery, foot-and-mouth disease, many diarrheas of children, milk 
sickness and the organisms of cholera infantum, and, rarely, Asiatic 
cholera as well, all may be transmitted through milk. 
Shell-fish, particularly oysters, have been known to transmit enteric 
diseases. This has been due, in the past, largely to the custom of 
exposing them in the estuaries of rivers where sewage flowed freely over 
them, to induce that highly unnatural condition known as "plump- 
ing." Typhoid bacilli enter the mantle cavity of the shell-fish, remain 
alive there and enter the digestive tract of man in a viable state when 
the shell-fish are consumed in an uncooked condition. 
Meats, particularly from beef and swine, have been known to 
transmit paratyphoid fever, botulismus (sausage poisoning) and 
meat poisoning as well. There is, in addition, a group of cases with 
somewhat insidious symptoms, which are probably due to the con- 
sumption of food, particularly meat, which has been decomposed by 
saprophytic bacteria. 
5. Animal Carriers. The microbic diseases which are transmissible 
to man from animals and from man to man by animals are varied 
in character. They comprise protozoan and bacterial infections 
and several of the so-called "filterable viruses." Of these diseases, 
comparatively few are common to man and animals. Microorganisms 
may be transmitted to man by animal carriers in at least four distinct 
ways : 
(a) By direct contact. 
(6) By indirect transfer. 
(c) By mechanical transfer. 
(f/) By intermediary hosts. 
