184 PUBLIC HEALTH BACTERIOLOGY 



protection was seen in regard to other eruptive and 

 non-eruptive fevers such as scarlatina, measles, typhus, 

 typhoid, and whooping-cough. On the contrary, some 

 specific diseases do not protect, but one attack seems to 

 render the individual more liable to another. Such are 

 diphtheria, pneumonia, influenza, gonorrhoea, erysipelas, 

 relapsing fever, and rheumatic fever. In all of these it is 

 probable that some degree of immunity results, but is of 

 very short duration, as has been definitely observed in 

 cholera and some other diseases. In all cases, however 

 short the immunity, it is absolutely specific against a 

 certain infective agent or its poison, and is not due to a 

 general increase of resistance. In fact the reduction of 

 the general resistance following the specific infection is 

 such as in some cases to predispose to other infections. 

 Thus, tuberculosis not infrequently follows a severe 

 attack of measles, whooping-cough, or typhoid. Recovery 

 from an acute infective disease is due to a process of 

 immunization going on during the progress *of the disease, 

 which at a certain point or stage is able to prevent the 

 further action of the infecting agent. The substances 

 formed do not always exterminate the virus from the 

 mucous surfaces ; and this is seen specially in typhoid 

 fever, where the recovered patient may continue to excrete 

 the living virus by the bowel or urinary discharges, and 

 in diphtheria, where the virus may persist in the throat. 



Artificial Immunity. Under this head may be classed 

 together forms (b) and (c) of acquired immunity. 



(b). By Active Immunization, or Protective Inoculation 

 where the specific protective substances have to be formed 

 in the body itself, as opposed to immunization by trans- 

 ference of protective substances formed by active 

 immunization in another animal, and called passive 

 immunization. In active immunization, the individual or 

 animal must undergo an infection followed by a reaction. 

 By this means the protective substances are formed, and 

 so the immunity is obtained only after the lapse of a period 

 of time, when the immunizing apparatus of the organism 

 is able to produce the protective substances in sufficient 

 amount. The immunity thus evoked is of a more 

 persistent type than that obtained by simply transferring 



