16 BOARD OF AGRICULTURE. 



an imponctrable mystery. If a man has had one attack of 

 small-pox, he is not only safe from ordinary exposures, but he 

 may, as a rule, risk contact with the worst forms of the dis- 

 ease. If a fowl has had one attack of the disease called fowl 

 cholera, it resists this disease when it is placed in the worst 

 infected flock ; it may cv^en be fed with the flesh of birds 

 which have died with the disease, or concentrated virus may 

 be introduced into its tissues, without producing an appre- 

 ciable eftcct. The greater part of these diseases are non- 

 recurrent, and inexplicable as this phenomenon may be, it is 

 unquestionable, and has been the bai?is of some of the most 

 eft'ective measures which have been proposed for the control 

 of these plagues. 



The earlier attempts to take advantage of the principle of 

 non-recurrence were crude and full of danger — i)cople were 

 inoculated with the unmitigMted small-pox virus, and the only 

 means of reaching: results more favorable than the averaije of 

 cases caused by natural methods of contagion was to select 

 virus from mild cases and to use it in small amount. Simi- 

 lar methods of inoculation for ovine variola or sliccp-pox 

 have long been practised in countries where this disease pre- 

 vails ; but with this plague as with human variola, it was al- 

 most an equal choice between the remedy and the disease. 

 The remedy, indeed, produced the disease with all its symp- 

 toms, and frequently in its severest fv)rm. The inoculation 

 which has long been practised in Europe for pleuro-pneumo- 

 nia or lung plague of cattle is an example of more I'avorable 

 results obtained by this same defective method ; but these re- 

 sults depend upon a very marked and interesting peculiarity 

 of this disease. The virus of pleuro-pnemnoni.1, owing to 

 some vital peculiarity which so far has evaded science, will 

 not invade the Avhole animal body as does the virus of most 

 other diseases. When the contagion occurs by natural chan- 

 nels the germs are drawn with the inspired air into the lungs, 

 they multiply locally and produce intense inflammation of 

 these organs and the membranes which cover them, but here 

 their action ends : if the chano-es in the lungs are not suflicient 

 to cause death, nature repairs the damage as well as possible 

 and the attack is over. If the virus is injected beneath the 

 skin of the neck, the germs again multiply locally, and now 



