86 SAPROPHYTISM, PARASITISM AND PATHOGEN ISM 
host and the development of those reciprocal reactions l^etween host 
and parasite, which collectively are included in the newly deAeloped 
science of Immunology. 
It would appear, therefore, that in their evolution toward parasit- 
ism, those bacteria which could thrive without producing deep-seated 
and rapid degradation of proteins, that is to say, whose metabolism 
approached more closely the intracellular metabolism of their host, 
would be the more adaptable to a parasitic existence, and this is in 
accord w4th what is known of the chemistry of these organisms. 
Their metabolism approaches rather closely that of their host. 
n. THE CYCLE OF PARASITISM. 
The cycle of parasitism for bacteria whose life cycle is such that 
but a limited excursion outside their host is possible for them— and 
this appears to be the case for the majority of organisms parasitic on 
man— consists of three separate and well-defined stages, as Theobald 
Smith^ has so clearly pointed out. They must— first— reach an 
appropriate host; secondly— multiply at least temporarily thereon, 
and thirdly— escape to other suitable hosts. Each phase of this 
parasitic existence must be exactly fulfilled, otherwise the cycle is 
broken and that particular strain dies out. It is not surprising, 
therefore, that the bacteria habitually parasitic for man are found 
variously upon the surface of the body— in the upper respiratory 
tract, the gastro-intestinal tract, or upon the mucous surfaces which 
are in direct communication with the exterior. Escape from the 
body of the host to other hosts is readily accomplished from these 
positions.^ 
Under special conditions, parasitic bacteria may actually invade 
the body of the host and become therefore temporarily pathogenic. 
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 
organisms is usually non-specific in character and sporadic in dis- 
tribution, 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 w^ere, 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 usu- 
ally 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, 
1 Theobald Smith: Loc. cit. - Ihid. 
