768 THE POPULAR SCIENCE MONTHLY. 



the other hand, is very prone to recur, and in not a few cases the origi- 

 nal attack merges into a chronic state of suffering, which may continue 

 for months or even years. 



I have taken scarlet fever as a representative of the class of infec- 

 tious diseases, the cause of which is- the contamination of the system 

 by some specific poison, and I have sketched in a few words the main 

 symptoms which result. For our present purposes the important points 

 are the contagious or infectious character of the disease, and the proofs 

 that the contagious material multiplies within the system which it has 

 invaded, and from which it sallies forth in quest of other victims. 

 There are, unfortunately, not a few diseases belonging to the same 

 category as scarlet fever, the principal being small-pox, measles, typhus, 

 influenza, whooping-cough, diphtheria, typhoid, and cholera. With 

 regard to all these it may be stated that they are all separate and dis- 

 tinct as regards causation. A case of scarlet fever never gives rise to 

 small-pox in those exposed to infection, neither does any one of the 

 above diseases ever pass into another. There are other subordinate 

 distinctions : the poison of scarlet fever, contained presumably in de- 

 tached particles of skin, clings for months to articles of clothing, espe- 

 cially woolen ones ; that of small-pox may be collected from the erup- 

 tion and preserved for years between pieces of glass ; that of typhus 

 is easily rendered innocuous by free ventilation. All these peculiari- 

 ties — and many more might be cited — point to important differences 

 in the nature of the infectious materials. 



What this infectious material really is has often been keenly de- 

 bated since medicine became a science, and at the present time is the 

 question which most closely occupies the minds of medical investiga- 

 tors. Merely to enumerate the inquiries, and to describe the experi- 

 ments and the theories based thereon, would fill a volume ; but it is 

 not to be wondered at that this subject should have excited so much 

 attention when we reflect upon the prevalence and fatality of the dis- 

 eases in question, and upon the comparatively slight influence which 

 treatment exercises upon their course. On the other hand, experience 

 clearly shows that their prevention is not only possible, but in some 

 cases easily accomplished. The knowledge of the causes of these dis- 

 eases would indicate the proper preventive measures, or at any rate the 

 direction which such measures should take, and hence a discovery of 

 the cause in any given case at once yields practical results. W T hen we 

 know what causes infection, we can apply disinfection with every pros- 

 pect of success. Without such knowledge success, if attained, must 

 be accidental rather than otherwise. The nature of the contagious 

 agencies, and the medium through which they spread, are the most 

 important points in connection with the subject of infection. 



There is strong evidence in support of the view that these contagia 

 are actual living things. Formerly the opinion was universally held 

 that infectious diseases were caused by foul air, and the effluvia con- 



