THE CYCLE OF PATHOGEN ISM 87 



Such an invasion is usually subsequent to a preexisting disease or to 

 local weakening of the tissues which under normal conditions suffice 

 to exclude these organisms. The disease produced by parasitic organ- 

 isms is usually non-specific in character and sporadic in distribution, 

 and ordinarily it does not attain epidemic proportions. The bacteria 

 which have penetrated into the tissues of the host are locked up there, 

 as it were, and their descendants cannot escape to other hosts, at least, 

 in numbers sufficient to perpetuate the invasive strain, for these 

 organisms have not perfected their pathogenic cycle. Parasitic 

 organisms, in other words, are " opportunists," as Theobald Smith 

 has admirably called them, rarely initiating disease, but usually able 

 to penetrate the body as secondary or terminal invaders. The colon 

 bacillus, for example, is an habitual parasite in the gastro-intestinal 

 tract of man and many animals. Under certain conditions it may 

 become invasive, causing cystitis, appendicitis, peritonitis, or other 

 inflammatory lesions, but it does not ordinarily become progressively 

 pathogenic for successive hosts, producing epidemics of cystitis, ap- 

 pendicitis or peritonitis. The staphylococcus is a common inhabitant 

 of the skin of healthy man. When the continuity of the epidermis is 

 destroyed, the organism may become invasive, causing furuncles, 

 osteomyelitis, or endocarditis. The pneumococcus is found in the 

 respiratory tract of many normal men, particularly in large cities, 

 where it exists as an "opportunist," ordinarily producing no harmful 

 effects, but frequently becoming invasive and producing a variety of 

 lesions when the general resistance of the host is lowered. 1 These 

 parasitic bacteria have not perfected their mechanism of entry into 

 the tissues of the host, and of escape from the tissues to the exterior, 

 consequently those strains which accidentally become invasive are 

 locked up in the body and, as a rule, either are overwhelmed by 

 thjeir host or perish with it. They are imperfectly pathogenic, in other 

 words. 



m. THE CYCLE OF PATHOGENISM. 



Habitually pathogenic bacteria those organisms which produce 

 progressive, specific disease from host to host actually invade the 

 living bodies of animals or man. This invasion may be direct* in which 

 event the microorganisms actually enter the tissues or body fluids 



1 Recent studies by Cole and his associates indicate that the ordinary "mou^" 

 pneumococcus differs serologically from the strains found in the saliva of pneumonl^ 

 cases. It is not improbable that similar serological differences may be demonstrated 

 in the group of the streptococci. 



