PLAGUE 165 



(4) Susceplibiliiy of llie host — i.e., :\ vulnerability of the living- 

 tissues by the bacteria. 



Increased by : Any constitutional depression, cold or wet, 

 starvation, youth of the patient, alcoholism, narccjtics and con- 

 stitutional diseases, e.g., Bright's disease or diabetes. 



(5) Entrance of the bacteria into the tissues, provided by : — 



(a) A wound or abrasion or ulcer, (b) An inflamed or injured 

 part, e.g., an inflamed tonsil or a sprained joint, (c) Pro- 

 found constitutional depression, allowing the normally 

 present bacteria to enter and multiply in tiie tissues. 



IMMUNITY. 



Is a condition of absolute or relative insusceptibility of the indivi- 

 dual to infection after invasion by virulent pathogenic bacteria. There 

 are two varieties : — • 



Natural Immunity. — Racial or individual. 



Acquired Immunity. — Active immunity; passive immunity. 



Natural Imiuuuily. — Nature of the condition is not known. No 

 antitoxic or antibacterial substance can be separated from the blood. 



Is lessened by cold or wet, starvation, youth, hcTmorrhage, 

 alcohol and poisons. 



Active Immunity. — ^Caused by a previous infection, either as the 

 natural or modified disease. 



Involves an actual illness. Takes time for its development. Lasts 

 many }'ears. 



Is brought about by : — 



(a) Previous attack. 



(b) Giving the disease, e.g., old-fashioned inoculation of small-pox. 



(c) Injection of attenuated virus, e.g., vaccination for small-pox, 

 anthrax (in cattle), hydrophobia. 



(d) Injection of dead bacteria, e.g., plague, typhoid, tuberculosis. 



(e) Injection of toxins, e.g., in the animals which provide the anti- 

 toxins of diphtheria and tetanus. 



Passive Iniinunity. — Caused bv injecting the blood of an actively- 

 immunized animal. Involves no illness. Follows immediately on 

 injection. Onlv lasts a few months. 



Is most successful against extracellular loxins, e.g-., diphtheria and 

 tetanus. 



PLAGUE. 

 DEFINITION. 



A septicaemia caused b\' the Bacillus pestis, spread amongst men 

 and other animals by fleas, characterized 1)\' fever, rapid course, and 

 high mortality. 



