THE CYCLE OF PATHOGEN ISM 87 
or other inflammatory lesions, hut it does not ordinarily hecome 
progressively pathogenic for successi\'e hosts, producing epidemics of 
cystitis, appendicitis or peritonitis. The staphylococcus is a common 
inhabitant of the skin of healthy man. When the continuity of the 
epidermis is destroyed, the organism ma\- become iuA'asive, causing 
furuncles, osteomyelitis, or endocarditis. Escape of the organisms 
thus locked up through channels in communication with the exterior is 
usually impossible, and such strains fail to reach other hosts and thus 
perpetuate the invasive strain. The non-specific types (Group IV) of 
pneumococcus are found in the respiratory tract of many normal men, 
particularly in large cities, where they exist as "opportunists" ordi- 
narily producing no harmful effects, but frequently becoming invasive 
and producing a variety of lesions when the general resistance of the 
host is lowered.^ 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 their 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 diseases (contagious) 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 and multiply there, or it may be indirect, in which instance 
their soluble toxins alone are absorbed by the host. The cycle of 
pathogenism, therefore, is more complex than the cycle of parasitism; 
it necessitates lodgment of the invading microbe on the body of the 
host, the location and penetration of the necessary portal of entry 
(which involves an initial skirmish between the organism and the 
non-specific natural defences of the host), growth within the tissues 
of the host in the presence of opposition there, escape from the tissues 
to the surfaces of the host or to some channel in communication with 
the exterior and, finally, the transmission of the organism, directly 
or indirectly, to other suitable hosts.- If the organism cannot force 
an entrance to the tissues of the host, that is, if the natural defences 
of the host suffice to keep out the prospective iuAader, the latter 
usually perishes and no infection takes place; if the organism does 
penetrate the tissues of the body, the invasion and growth of the 
1 Studies by Cole and his associates indicate that the ordinary "mouth" pneumococcus 
differs serologically from the strains found in the saliva of pneumonia cases. The 
latter appear to possess the power of independent invasiveness, and consequently to 
belong to the grouj) of "progressively pathogenic bacteria." It is not improbable that 
similar serological differences may be demonstrated in the group of the streptococci. 
2 For illustrative examples see Theobald Smith: Jour. Am. Med. Assn., 1917, 68, 
669, 764. 
