70 Causes of Disease. 



when ulceration has occurred in the primary infection and the 

 protective epitheHum has been lost, or when in the course of a 

 disease the bactericidal substances normally present in the fluids 

 of the body have been exhausted and the resistance of the tissues 

 reduced in consequence. Not infrequently such mixed infection is 

 to be seen in hog-cholera, the micro-organisms of which cause 

 necrotic and coagulative destructive changes in the intestinal 

 mucous membrane and general wasting; there is thus prepared so 

 suitable a culture medium for the sputum bacteria and necrosing 

 bacilli present in the pharyngeal mucus and in the intestinal con- 

 tents that these readily penetrate into the tissues and materially 

 contribute to intensify the disease and hasten a fatal termination. 

 Admixture with or contamination (association) of an infection by 

 one or more other types of pathogenic microbes, each with its 

 special properties, and the combined action of all, as a rule causes a 

 severe course of the infectious disease and occasions unusual 

 pathological processes {compUcations). 



The mere presence and microscopic recognition of several types of 

 microbes in any focus of disease or in the tissues of the cadaver by no 

 means warrant, without other knowledge, the assumption that there has 

 existed a mixed infection ; after the death of the animal great numbers 

 of saprophytic organisms living on the surface of the mucous membranes 

 wander into the organs, and necrotic or ulcerating foci in the lungs, intestine 

 or skin are apt to harbor the same sort of organisms from the air or food 

 (so-called symbiotic bacteria). The association of such essentially non- 

 pathogenic germs, or, too, of pathogenic microbes with another pathogenic 

 variety, may (as especially pointed by Leclainche-Vallee in studies on 

 symptomatic anthrax) determine actual infection ; for example, a symbiotic 

 bacterium may prevent phagocytosis by a negative chemotactic power and 

 thus produce conditions favoring the increase of the other infecting agent. 

 This should not be interpreted as meaning that a given microorganism 

 cannot alone cause its special disease, and that only when in association 

 with other microbes can produce its effect ; it merely applies to special 

 conditions in which the specific infectious agent is attenuated or is situated 

 in an unsuitable point for infection. All known pathogenic microorganisms 

 are individually capable of giving origin to their peculiar infections, and 

 each is the specific bearer of such infection. 



Those micro-organisms regarded as pathogenic may be classed 

 in two groups : In the first group are microbes usually living in 

 the external world, but inducing disease in the animal body if by 

 accident they gain access to it (ectoge)ious infectious agents, 

 facultative parasitic microbes). The diseases arising from this 

 group affect individual animals here and there, or may attack a 



