388 ELEMENTARY BIOLOGY 



In all essentials the methods of infection and transmission 

 of disease are the same for the domestic animals as they 

 are for man. When we consider that disease is preventable 

 just in proportion as we understand the causes and the 

 modes of infection, we may well believe with certain special- 

 ists that the study of bacteriology is among the most im- 

 portant contributions of the nineteenth century to the welfare 

 of the human race. In Fig. 202 are given the annual losses 

 due to various diseases, and an indication of the extent to 

 which these may be prevented. 



430. Protection against infection. The chief means of pre- 

 venting infection consists of preventing the contamination of 

 our food by bacteria. This means that pains must be taken as 

 to the exposure of fresh food to dust, to the mouth-spray of 

 people or other animals, and to contact with unclean hands 

 or with containers of all kinds. Many cities now require that 

 all food exposed for sale, such as meat, pastry, confectionery, 

 and the like, be covered against dust as well as against the 

 visits of insects ; but fruit and vegetables are still commonly 

 exposed, at least to dust. In the case of fresh fruits or vege- 

 tables the peel is usually a sufficient protection against bacteria. 

 But the peel of many vegetables contains very desirable food 

 material, which should not be thrown away. Fruits and vege- 

 tables that are cooked are generally safe, since the cooking itself 

 kills the bacteria (see p. 112). But lettuce, celery, and other 

 vegetables that are eaten without cooking have frequently been 

 the means of infecting people with disease, since bacteria in the 

 soil may cling to the plants, and some of the disease-causing 

 bacteria may get into the soil of gardens. Such plants should 

 be thoroughly washed before being used as food. 



431. Care of food. The fact that food rots so readily when 

 left to itself shows that it contains the materials necessary to 

 maintain the life of bacteria. We should therefore keep it 

 under conditions that are not favorable to the growth of these 

 organisms. We have the practical choice between keeping our 



