Contagious (^Zymotic) . Diseases . 13 



and disinfection is to be preferred when it can be availed of at 

 reasonable expense. 



Habits of Microbes as affecting Suppression. 



As bearing on the suppression and extinction of contagious 

 diseases, the life history of each pathogenic microbe, and partic- 

 ularly its survival as a saprophyte outside the bodj^ or its strict 

 confinement to a life in the animal body (pathogenic career) 

 offers most valuable indications. The microbe which is restricted 

 to an intracorporeal existence in the animal body can be certainly 

 and definitely circumscribed and exterminated, and a disease 

 which has prevailed and decimated a race of animals from the 

 dawn of. their existence can be wiped out from the face of the 

 earth. A microbe which can live and multiply not only in the 

 body of an animal host, but also outside in a decomposing organic 

 matter (a saprophyte), can not be met and exterminated on the 

 same easy and certain terms, but must be dealt with in all parts 

 of the outside world wherever there is a concurrence of the suit- 

 able dead organic matter, and the pathogenic microbe. It may 

 or may not be possible to utterly exterminate a microbe which 

 possesses this habit of life, but we can usually circumscribe its 

 prevalence and shut it out of given areas, by eliminating it from 

 the animal system within such areas, and putting an end within 

 the protected field to the conditions that favor its saprophytic life. 

 Again, a microbe which is pathogenic when introduced into the 

 animal body, and which may live outside that body as a sapro- 

 phyte, under conditions which are unusual and which can be suc- 

 cessfully altered, stands midway between the two first named as 

 regards its amenability to suppression and extinction. Again, 

 certain microbes, though deadly to their animal host, habitually 

 employ ticks or predatory insects to convey them from one animal 

 host to another. These can be cut short in their pathogenic 

 career by the simple expedient of destroying or excluding the in- 

 vertebrate host which acts as a bearer between the two higher 

 animals. 



