88 SAPROPHYTiSM, PARASITISM AND PATHOGEN ISM 
microorganism leads to disturbances of structure, function or com- 
position of the host, which are abnormal and inimical to his well-being. 
The production of disease, therefore, depends ordinarily upon the 
ability of the microorganism to multiply in the tissues or the body 
fluids of the host. Bacteria, therefore, which cannot force an entrance 
into the tissues of the host, multiply there and escape to the exterior 
and eventually to other susceptible hosts do not produce progressive 
(or contagious) disease. 
The nature and extejit of the disease produced by progressively 
invasive, or pathogenic microbes, depend upon several factors: (1) 
The kind of microorganism; (2) the number of microorganisms; (3) 
their ability to locate and force an entrance to the tissues of the body 
(their virulence, in other words) ; (4) the location and extent of their 
multiplication in the tissues of the host ; (5) the response of the tissues 
of the host to this invasion, and (6) the nature and extent of the 
specific offense of the host in response to the invasion. 
The contagiousness of a disease depends upon the ability of the 
invading organisms to escape from their host in sufficient numbers 
to infect new hosts and -to survive environmental vicissitudes until 
new hosts are reached. A few examples will indicate the principal 
variants of the pathogenic cycle commonly met with among progres- 
sively pathogenic bacteria. 
The tubercle bacillus ordinarily gains entrance to the host through 
the air passages. The organisms pass through to the alveoli of the 
lungs, set up infection there, and gradually are shut off from com- 
munication with the exterior through the formation of the tubercle. 
Eventually, after a longer or shorter time, these tubercles break down, 
typically into the air passages— and discharge there large numbers 
of tubercle bacilli. These are coughed up by the patient and are 
eliminated from the body, usually in enormous numbers, by droplets 
and in the sputum. Pulmonary tuberculosis is typically a chronic, 
focal disease. The perpetuation of the tubercle bacilli is assured 
through their elimination from the diseased body in enormous num- 
bers through long periods of time, their ability to resist desiccation, 
and the relative directness with which they reach other hosts. 
The typhoid bacillus gains entrance to the body through the mouth 
and the intestinal tract. The organisms penetrate the intestinal 
mucosa, develop in the internal organs, particularly the spleen, and 
after a rather definite excursion in the tissues of the body, enter into 
the intestinal tract again, either through ulcers or the gall-bladder or, 
occasionally, they appear in the urine. They are ehminated from 
the body in great numbers, either with the feces, or less commonly, 
the urine, and they gain access immediately to other subjects through 
direct contact, or more or less indirectly through water or food, in 
sufficient numbers to set up infection in at least some of them. 
The gonococcus is transmitted directly by contact. Occasionally 
the infection may be somewhat less direct, involving the conjunctiva. 
