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been given to the subject of preventing the spread of infectious diseases 

 by means of notification, isolation, disinfection and vaccination, and still 

 more recent methods of treatment and prevention by means of the tak- 

 ing of cultures and the use of antitoxin, the death-rate from infectious 

 diseases has been sensibly diminished, and the length of human life cor- 

 respondingly prolonged, and this is notably true of England, the country 

 where the most careful attention has been given to the subject and the 

 greatest amount of money expended in its accomplishment. 



Public hygiene or preventive medicine again is like agriculture in its 

 method of dealing with those evils which, on the one hand, destroy 

 human beings and limit their progress, and on the other, those which 

 seriously interfere with the abundance and the quality of growing crops, 

 and the principles of prevention which are applied in either case are 

 very much alike. 



If a sound, healthy infant, born of healthy parents, were to be placed 

 in a glass case, and fed with pure food which had been freed from all 

 germs of disease by due process of sterilization, and were constantly sup- 

 plied with pure air which had also been sterilized ; if the water which 

 it drank were to be always pure spring water, and if in all other points 

 it were to be treated on perfectly healthful principles, such an infant 

 would never die of measles nor small-pox nor scarlet fever nor typhoid 

 fever nor whooping cough nor consumption. 



So, also, in agriculture, if 'an apple tree or a peach tree were to be 

 enclosed in a glass case, where it would be supplied with abundance 

 of sunlight, with filtered water and sterilized air and soil deprived 

 of all pathogenic germs or eggs of noxious insects. No canker worm 

 nor caterpillar nor gypsy moth nor any other pest could possibly molest 

 it, and its leaves and flowers and fruit would mature and ripen in fair- 

 ness and beauty. This is the principle of isolation. 



There is also a great similarity in the methods of spread of infectious 

 diseases and of insect pests, and while there is a similarity in the general 

 group of infectious diseases to that of insect pests, there are also many 

 points of specific difference. 



Influenza, for example, spreads with amazing rapidity and attacks 

 great tracts of country in a few hours' time. It appeared in Boston 

 about Dec. 19, 1889, and in less than a week had also appeared in nearly 

 every city of the northern States. One class of diseases, cholera and 

 typhoid fever, spreads through the medium of water supplies, another 

 class, including small-pox and scarlet fever, by means of the air and by 

 actual contact. The spread of consumption is favored by the presence 

 of dust diffused through the air of rooms and carrying with it the germs 

 of disease. 



So, too, in agriculture, the various insect pests differ in the method of 

 their spread. The female canker worm ascends the trunks of trees in 

 the warm days of late autumn or early spring and lays her eggs on the 

 twigs, to be hatched in the months of May or June. Hence the mode 

 of prevention is to place a barrier upon the trunks of the trees which 

 shall hinder the insects from gaining access to the branches. So with 

 the American tent caterpillar. This insect lays its eggs upon the small 



