274 THE INFECTIOUS DISEASES. 



Secondary lesions are diffuse suppurative inflammation of the skin, 

 congestion, inflammation and ulceratiou of the mucous membranes, 

 haemorrhages in various parts of the body, swelling and ulceration of the 

 lymphatic tissues, albuminous degeneration of the kidney, liver, and 

 spleen, and leucocytosis. 



The skin lesion shows in general at first circumscribed areas of in- 

 flammation above the ends of the papillae, with the development of a 

 fluid-filled reticulum, so that vesicles more or less umbilicated are formed 

 (Fig. 138). These at first contain a clear fluid, but by the gathering of 

 pus cells the fluid becomes turbid and accumulates to form a pustule. 

 Haud-in-hand with these changes the papillee and adjacent layers of the 

 coriurn may become infiltrated with cells. The contents of the pustules 

 and the necrotic tissue above dry and form the crusts. When the changes 

 are largely confined to the epidermis the lesion may leave no deformity. 

 But if the changes in the cutis are considerable, cicatricial tissue may 

 form, leaving scars. The association of local haemorrhage with the above 

 changes gives rise to the hsemorrhagic form of exantl^ein. 



ARTIFICIAL IMMUNIZATION IN SMALLPOX. 



Smallpox affords one of the most striking examples of positive and 

 prolonged acquired immunity conferred by a successfully weathered 

 attack of an infectious disease. 



In the early days attempts were made to mitigate the virulence of 

 smallpox acquired by exposure in the usual ways by artificial inoculation 

 of material virus taken from a smallpox pustule. The usually rela- 

 tively mild form of the disease induced in this way also conferred immu- 

 nity. But the individual was during his immunization a source of danger 

 to others. The great discovery of Jenner that by inoculation with virus 

 from cowpox immunity was secured against smallpox, need not be con- 

 sidered in detail here. The immunity secured in this way, while not 

 absolute, is usually effective and involves as a rule but slight indisposi- 

 tion. By revacciuation after an interval of a few years practical protec- 

 tion is secured, so that the occurrence of smallpox epidemics to-day is 

 possible only through neglect of simple and positive protective measures. 



The more recent view of the immunity conferred by vaccination 

 against smallpox is based upon the demonstration that the disease variola 

 in man and the disease vaccinia in the bovine species are of the same 

 nature and not different, as was formerly believed. This has been estab- 

 lished by numerous inoculation experiments. The disease in the cow is 

 a modified form of the human disease. The effect of the passage of the 

 unknown micro-organisms through the insusceptible bovine thus runs 

 the rationale in the new light is so to diminish the virulence of the 

 germ that by its subsequent inoculation in man immunity is secured with- 

 out the profound disturbance which infection with a germ of unmitigated 

 virulence would involve. ' 



l ln diphtheria the perfection of the process of artificial immunization and the estab- 

 lishment of a precise and successful curative method are the direct results of a long, pa- 

 tient, logical series of animal experiments with a definite end in view, and by the use 



