PARASITIC AND PATHOGENIC BACTERIA 91 



The majority of these organisms, however, particularly the coccal 

 forms, as the staphylococcus, streptococcus and pneumococcus 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 bojdy 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 presumably 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 occasionally 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 "opportunist" 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. Air-borne Infection. Bacteria which cause progressive disease, 

 particularly of the respiratory tract, are discharged from the diseased 

 body principally through the mouth and nose and find lodgment in the 

 environment of the patient through the medium of the air, from whence 

 they settle upon various substances, as food, clothing, and walls and 

 floors of rooms. These bacteria probably do not proliferate to any 

 extent outside of the body, but they resist drying and may remain 

 fully virulent for considerable periods of time and potentially able to 

 infect a certain proportion of those individuals who may be exposed 

 to them. 



These air-borne infections are transmitted in at least two rather 

 distinct ways: (a) by dust, and (6) by droplet infection. 



(a) Organisms which are transmissible through dust must first 

 of all be able to survive considerable periods of drying. The larger 

 particles of dust to which bacteria may become attached soon settle 



