54 INFECTION AND IMMUNITY. 



tion and light, as in tuberculosis, the importance 

 of dust infection approximates, though it may not 

 equal, that of droplet infection. 



^ US ^ as ^ ie a * r * s ^ ie P r i nc iP a l medium of con- 

 Mediun veyance for the group of diseases discussed above/ 

 so contaminated water plays an important, but not 

 the sole, part in the transmission of another group. 

 Typhoid and paratyphoid fever, cholera and bacil- 

 lary dysentery are the chief representatives. 



Epidemics. which arise in this way are frequently 

 spoken of as "water-borne" epidemics. Sometimes 

 typhoid and cholera are called "water-borne" dis- 

 eases, but epidemics are so often instituted and 

 maintained by various kinds of indirect contact 

 that the appellation is one-sided. 



condition*. Three essential conditions are required in order 

 that a disease may be more or less habitually 

 transmitted through water: First, the discharge 

 of the organisms from the body of the patient in 

 such form that they may reach a water supply. 

 Second, the ability of the organisms either to live 

 for a moderate length of time in the water, or to 

 proliferate in it, without losing virulence. Third, 

 the utilization of the gastro-intestinal tract, the 

 upper respiratory tract, or the lungs indirectly 

 from the latter, as an atrium of infection. 



The first condition is readily realized in the dis- 

 eases mentioned, inasmuch as the micro-organisms 

 are discharged in large numbers with the feces, 

 and also, in the case of typhoid and paratyphoid, 

 with the urine. 

 sources of All types of water supplies may be contaminated 



Contamina- ,, , . ^TTI , i * 



tion. by these excretions. when the water of a com- 

 munity is taken from a stream the latter may be 

 infected by the sewage of another community 



