524 MICROBIOLOGY OF THE DISEASES OF MAN AND ANIMALS. 



of the body the following may be said. For a long time it was claimed 

 that the internal organs of man and animals were sterile. Neisser is 

 authority for the statement that the internal organs of healthy animals 

 are sterile. This, however, has been shown not to be the case univer- 

 sally. Bact. tuberculosis has been found in normal human and bovine 

 lymph glands. The various pus-producing micrococci have been fre- 

 quently found in the spleen, kidney, and liver. Perhaps the commonest 

 group of bacteria to be isolated from the internal organs are the intestinal 

 forms. Peristaltic action and the diffusion of food through the intestinal 

 wall may be influencing factors. The fact that the internal organs are 

 not sterile in every case is important as it may account for the so-called 

 autogenic infections. 



THE MANNER IN WHICH INFECTIOUS AGENTS ENTER THE BODY AND 



THEIR SOURCE. 



AIR-BORNE INFECTIONS. The causal microorganisms of infectious 

 diseases are frequently excreted from the body of the diseased individual 

 and are deposited on the clothing, furnishings, on the floors and walls, or 

 on the ground. These microorganisms probably do not proliferate 

 except in rare instances, but frequently remain virulent and capable of 

 being carried through the air and producing disease in others individuals. 

 It is generally believed that in diseases such as smallpox, measles, 

 scarlet fever and the other acute exanthematous diseases, together with 

 such diseases as bubonic plague and diphtheria, the infectious agents 

 may be carried through the air after having been deposited on clothing 

 and furnishings, but this view is not held by some specialists. 



It has been supposed that the only way that bacteria could be 

 carried in the air was after having been dried on particles of dust and 

 carried by currents of air. This, however, has been shown not always 

 to be the case and we now know that infectious microorganisms may 

 be carried on small particles or droplets of sputum or moisture. These 

 two types of aerial infection are known respectively as dust and droplet 

 infection. They will be given a brief consideration. 



Dust Infection. Infectious microorganisms to remain virulent and 

 be able to produce infection must successfully resist drying after 

 being affixed to particles of dust. After being dried the particles are 

 frequently moved and whirled about by air currents. The larger particles 



