90 SAPROPHYTISM, PARASITISM AND PATHOGENISM 
environment makes it almost inevitable that microbes shall collect 
there. It is quite probable, however, that the larf2;er niunber of micro- 
organisms which reach the skin are not only non-pathogenic, they are 
not even habitually parasitic. Most of them are found there only 
transiently. Certain organisms, however, occur among these adven- 
titious microbes, which appear to be habitual parasites, and many of 
these bacteria, under certain conditions, produce disease. Of these. 
Staphylococcus aureus and albus and Streptococcus pyogenes are 
almost invariably present not only on the skin, but on the exposed 
mucous membranes, particularly those of the nose and throat. The 
influenza bacillus, diphtheria bacillus, the pneumococcus, particularly 
members of the non-specific Group IV, and even the tubercle bacillus, 
meningococcus and other organisms may also be occasionally thus 
found, particularly in the nose and throat of healthy men. The 
occurrence of these organisms is readily explained ; the secretions of the 
nose and throat, as well as that of the skin are excellent culture media 
for these organisms, which collect at these sites and grow upon the 
various secretions and desquamated cells. 
The majority of these organisms, how^ever, particularly the coccal 
forms, as the staphylococcus, streptococcus and pneumococcus (Type 
IV) are to be regarded as "opportunists;" they do not of themselves 
initiate disease, as a rule. They are to be regarded rather, as Theobald 
Smith has called them, "organisms of the diseased state," because of 
their invasion of the body secondary to other intercurrent diseases. 
Even the tubercle bacillus and the diphtheria bacillus, particularly 
the latter have been found in the mouths of men who apparently* 
have had neither tuberculosis nor diphtheria, ^ yet these organisms 
appear to be virulent when tested in the usual manner and presum- 
ably might be able to incite disease whenever conditions favor their 
entrance to the tissues of the body. Theoretically at least, people 
who harbor these organisms are potential sources of danger to others. 
Even the internal organs of healthy individuals may contain parasitic 
bacteria without harm, although these organisms naturally are not 
present in large numbers. Tubercle bacilli have been found occa- 
sionally in lymph glands in normal man and in cattle.^ Intestinal 
bacteria also occur not infrequently in the apparently healthy tissues 
of the body. In rare instances, B. coli may be present in the urinary 
bladder without causing noteworthy symptoms. 
B. How Pathogenic Bacteria Reach the Body.— The manner in 
which bacteria of the "opportiniist" type reach the body has been 
considered above. It is now- necessary to consider the manner in 
which bacteria which cause progressive disease from man to man 
reach the body. 
1 Guthrie, Gelien and Moss, however, have shown that many of the diphtheria strains 
thus found do not produce toxin. 
2 Harbitz: Jour. Inf. Dis., 1905, 2, 143. 
