4 VACCINE AND SEKUM THERAPY 



nocuous, whereas, if gaining an entrance through the 

 skin, they set up disease. Conversely, typhoid germs 

 may be rubbed into the skin, and may only set up a 

 slight local reaction, whereas, if swallowed by man, 

 they give rise to a dangerous disease. Animals are 

 protected against bacterial invasion in various ways. 

 Externally the body is guarded by its covering of 

 skin and mucous membrane. When these are healthy 

 and undisturbed, micro-organisms are usually held at 

 bay, with exception, perhaps, of a few organisms such 

 as tubercle bacilli, which may pass through the 

 intestinal mucosa into the lymphatics, without causing 

 local lesions. Even when bacteria gain entrance to 

 the body by their most favoured channel, they are 

 not always capable of producing disease. The bodies 

 of animals have at their disposal certain general 

 systematic weapons of defence, both in the blood 

 serum and the cellular elements of blood and tissues, 

 which, if normally vigorous and active, will usually 

 overcome a certain number of the invading germs. 

 If these defences are depressed, or the organism highly 

 virulent, infection will take place. The disease pro- 

 cesses arising as the result of bacterial invasion may 

 depend wholly or in part upon the mechanical injury 

 produced by the process of inflammation, the disturb- 

 ance of function caused by the presence of the bacteria 

 in the capillaries and tissue spaces, and the absorption 

 of the necrotic products, resulting from the reaction 

 between the body cells and the micro-organisms. To 

 a large extent, however, infectious diseases are 



