ORIGIN OF DISEASES. 525 



atmosphere of an unventilated drain is very suitable to their growth. If 

 the air of an unventilated drain is admitted into a house or stable the 

 germs of disease may come with it. Water, into which sewage has per- 

 colated, is also very favorable to the development and gro\vth of germs 

 of disease. If water, tainted by sewage, is drunk by man or beast, the 

 germs of disease will probably very soon make their presence felt. 



1053. Of the Spreadhig of Specific Diseases. 



The question is often asked, why, if specific disease be due to local 

 causes, are not all the men or all the animals subject to those causes 

 affected? A similar question with equal reason might be asked why 

 certain seeds germinate readily in certain soils under certain climatic 

 influences ; and why, in similar soils under different climatic influences 

 they do not germinate. A similar question might also be asked of the 

 advocates of the theoiy of the propagation of disease by infection and 

 contagion. 



The answer to these questions is the same. Different systems are not 

 equally liable to be affected. Nay more, the state of the system may 

 vary from day to day, even from hour to hour. A man or animal in good 

 health, and with a good constitution, may long resist malarial influence. 

 A delicate ailing susceptible system may soon be affected. Again, a man 

 may be strong and hearty in the morning. His system may be able to 

 resist noxious influences. He may return home hungry and fatigued. 

 He may become susceptible. Again, he may resist malaria for days or 

 weeks. His system may become lowered from a variety of causes, ren- 

 dering him susceptible to malarious influence. A seed bed may be dry 

 and hard. Seeds will not germinate. A shower of rain may come, fol- 

 lowed by a warm sun, and soon the seeds will germinate, 



A simple illustration may perhaps help to explain what we mean. 

 Take half a dozen sheets of paper, — one of the best glazed, another of 

 inferior quality, another rough, another of whited brown paper. Hang 

 them up in a draught in a London fog for a few hours. Note the amount 

 of " blacks " which have adhered to each. To the best glazed few, if 

 any, " blacks " will be found adhering, and these will be easily blown off. 

 More blacks will be found adhering to the inferior paper ; still more to 

 the rough paper, and it will not be easy to dislodge them ; whilst to the 

 poor whited brown paper almost every black will be adhering. 



Now let us reduce the state of " health." Suppose the day changes 

 from dry to damp. Examine the papers. Even the best glazed paper 

 will have become somewhat moist, and many blacks will be adhering to 

 it and will not be easily blown off ; and so in increasing proportion, until 

 we come to the whited brown paper, which has absorbed all the moisture, 

 and in which every black will be sticking fast, and cannot be dislodged. 



All however have suffered from lowering of tone. And so it is in regard 

 to men and animals subjected to noxious influences. 



The germs or particles may find a suitable nidus in one system, which 

 they do not find in another ; and the conditions of the system in which 



