88 SAPROPHYTISM, PARASITISM, AND PATHOGENISM 



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 

 lodgement 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 invader, 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 microorganism leads to 

 disturbances of structure, function or composition 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 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 disease. 



The nature and extent of the disease produced depends upon several 

 factors: (1) the kind of microorganism; (2) the number of micro- 

 organisms; (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 secondary, specific defense 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 vari- 

 ants of the pathogenic cycle commonly met with among progressively 

 pathogenic bacteria. 



The tubercle bacillus ordinarily gains entrance to the host through 

 the air passages. The organisms pass through the alveoli of the lungs, 

 set up infection there, and gradually are shut off from communication 

 with the exterior through the formation of the tubercle. After a 



