RELATION OF BACTERIA TO DISEASE. 99 



mucous membranes of the respiratory and digestive 

 tract. In these situations pathogenic bacteria of slight 

 virulence are always present even in health. Thus 

 in the upper air-passages there are usually found strep- 

 tococci, staphylococci, andpneumococci. When through 

 a cold, or the invasion of another infective agent, as 

 the diphtheria bacillus, the epithelium of the mucous 

 membrane of the throat is injured or destroyed, the pyo- 

 genic cocci already present are now enabled in this dis- 

 eased membrane to grow, produce their poison, and 

 even invade deeper tissues. The intestinal mucous 

 membrane is invaded in a similar way by the colon 

 bacilli and other organisms after injury by the typhoid 

 bacilli or cholera spirilla. Generally speaking, all in- 

 flammations of the mucous membranes contain some of 

 the elements of mixed infection. Blood infection, on 

 the other hand, is usually due to one form of bacteria, 

 as even when several varieties are introduced, only 

 one, as a rule, is capable of development. The same 

 is true to a somewhat less extent of inflammation of 

 the connective tissue. The additional poison given off 

 by the associated bacteria aid infection by causing a 

 lowering of the vital resistance of the body. 



The bacteria are also at times directly influenced by 

 the products of associated organisms. These may 

 affect them injuriously, as, for example, the pyogenic 

 cocci in anthrax; or they may be necessary to their 

 development, as in the case of anaerobic bacteria. JSTot 

 infrequently the tetanus bacilli or spores would not be 

 able to develop in wounds were it not for the presence 

 of aerobic bacteria introduced with them. This is shown 

 outside the body, where tetanus bacilli will not grow in 

 the presence of oxygen unless aerobic bacteria are asso- 



