236 PATHOGENIC BACTERIA OUTSIDE THE BODY 



The fact that such individuals are known to have had the 

 disease is a guide for control. The methods to be used are 

 essentially the same as for the sick, (a) though obviously 

 such human carriers are much more difficult to deal with 

 since they are well. 



(c) Another class of carriers is those who have never had 

 the disease. Such individuals are common and are very 

 dangerous sources of infection. Many of them have asso- 

 ciated ivith the sick or with convalescents, and these should 

 always be suspected of harboring the organisms. Their 

 control differs in no way from that of class (b). Unfortu- 

 nately a history of such association is too often not available. 

 Modern transportation and modern social habits are largely 

 responsible for the nearly universal distribution of this type 

 of carrier. Their detection is probably the largest single 

 problem in the prevention of infectious diseases. A partial 

 solution would be universal bacteriological examination. 

 In our present stage of progress this is impossible and would 

 not detect carriers of diseases of unknown cause. 



The various classes of carriers just discussed are in a 

 large part responsible for the continued presence of the 

 commoner diseases throughout the country. The difficulties 

 in control have been mentioned. A complete solution 

 of the problem is not yet obtained. The army experience 

 of the past few years in the control of infectious diseases 

 shows what may be done. 



There is another class of carriers which might be called 

 the ''universal carrier," i. e., there are certain organisms 

 which seem to be constantly or almost constantly present 

 in or on the human body. These are staphylococci, strepto- 

 cocci and pneumococci, all Gram-positive organisms. They 

 are ordinarily harmless parasites, but on occasion may give 

 rise to serious, even fatal, infection. Infected wounds, 

 pimples, boils, ''common colds," most "sore throats," 

 bronchitis, pneumonia are pathological conditions that come 

 in this class. Such infections are usually autogenous. 

 There is a constant interchange of these organisms among 

 individuals closely associated, so that all of a group usually 

 harbor the same type though no one individual can be 



