PATHOGENIC BACTERIA OUTSIDE THE BODY 235 



animals, etc. The mode of limiting this class of diseases 

 is evidently to isolate the sick, disinfect their discharges 

 and their immediate surroundings, sterilize such products 

 as must be handled or used, kill lower animals that are 

 dangerous, and disinfect, bury properly or destroy their 

 carcasses. 



Classes of the sick that are especially dangerous for the 

 spread of disease are the mild cases and the undetected cases. 

 These individuals do not come under observation and hence 

 not under control. 



(6) This class of carriers offers a difficult problem in the 

 prevention of infectious diseases, since they may continue to 

 give off the organisms indefinitely and thus infect others. 

 Typhoid carriers have been known to do so for fifty-five 

 years. Cholera, diphtheria, meningitis and other carriers 

 are well known in human practice. Carriers among animals 

 have not been so frequently demonstrated, but there is 

 every reason for thinking that hog cholera, distemper, roup, 

 influenza and other carriers are common. Carriers furnish 

 the explanation for many of the so-called "spontaneous" 

 outbreaks of disease among men and animals. 



It is the general rule that those who are sick cease to carry 

 the organisms on recovery, and it is the occasional ones who 

 do not that are the exceptions. In those diseases in which 

 the organism is known it can be determined by examination 

 of the patient or his discharges how long he continues to 

 give off the causative agent. In those in which the cause 

 is unknown (in human beings, the commonest and most 

 easily transmitted diseases, scarlet fever, measles, German 

 measles, mumps, chicken-pox, small-pox, influenza) no such 

 check is possible. It is not known how long such individ- 

 uals remain carriers. Hence isolation and quarantine of 

 such convalescents are based partly on experience and partly 

 on theory. It is highly probable that in the diseases just 

 mentioned transmission occurs in the early stages only, 

 except in small-pox and chicken-pox, where the organism 

 seems to be in the pustules and transmission by means of 

 material from these is possible, though only, by direct con- 

 tact with it. 



