BACTERIA AND HEALTH 391 



which milk becomes contaminated. The conditions under 

 which the cows live make it almost impossible to prevent 

 the hairs and skin of the animal from becoming the bearers 

 of bacteria of many kinds. While the milk in the udder of 

 the cow may be quite free of any contamination, by the time 

 the milk has been poured from the pail to the can it is sure 

 to have many bacteria floating in it. The high temperature 

 makes the multiplication of the organisms proceed very rapidly. 

 By the time the milk is ready for delivery in the city, it con- 

 tains a large number of bacteria in every drop. 



On page 127 are given the rules for the care of milk in- 

 tended for city markets. There is a biological reason for every 

 rule given, and this should be clear to every student of the sub- 

 ject. It has been found practically impossible to obtain milk 

 in large quantities without excessive numbers of bacteria. For 

 this reason the practice of pasteurization has come into more 

 and more general use. This consists of raising the temperature 

 of the milk to about 140-! 5 5 F., and keeping it there for 

 from ten to twenty minutes. Pasteurization does not, of course, 

 remove the bacteria ; it only kills them. 



433. Water supply. Next to milk, the water supply is 

 perhaps the source of greatest danger to the community. 

 In towns and cities that still depend upon separate wells or 

 springs for water the amount of sickness and the proportion 

 of deaths is likely to be much higher than in such places as 

 have a central water supply. To be sure, if the central water 

 supply becomes contaminated, more people are likely to be 

 injured in a short time. But it is easier to control the sani- 

 tary condition of one large reservoir than that of hundreds of 

 wells. The bacilli of typhoid fever will remain alive in water 

 for two or three weeks, and are the most frequent disease 

 germs transmitted by water. But other diseases may also be 

 transmitted in this way. The diagram in Fig. 203 shows the 

 reduction in disease and death that was brought about by 

 improving the water supply in the state of New York. 



