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, and pneumococci. 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 anaérobic bacteria. Not 
infrequently the tetanus bacilli or spores would not be 
able to develop in wounds were it not for the presence 
of aérobic bacteria introduced with them. This is shown 
outside the body, where tetanus bacilli will not grow in 
the presence of oxygen unless aérobic bacteria are asso- 
