THE PATHOGENIC PROTOZOA 155 



Yellow fever is inoculated by a mosquito, Stegomyia fasdata. 

 These diseases have all the behaviour of protozoal infections. 

 By reason of certain procedures of immunization which are 

 common to them all, horse-sickness, cattle-plague, and hog- 

 cholera present some points of similarity. 



3. The diseases characterized by localization to, and lesions 

 in, the epithelium, for example, small-pox and vaccinia, foot- 

 and-mouth disease, molluscum contagiosum of birds and man, 

 scarlatina, the jaundice of silk-worms, and trachoma or granular 

 conjunctivitis are perhaps to be classed in this group. 



Small-pox or vaccinia is the type-specimen of these infections ; 

 the characteristic lesion is the pustule and the pustule is a 

 collection of epithelial cells containing the virus and forming a 

 focus of culture in vivo. The cells which build up the little 

 tumour have no longer the normal structure of the epidermic 

 or Malpighian cells ; they have become globular, voluminous 

 and " dropsical " ; the nucleus is altered, being swollen and 

 frequently out of position ; beside the nucleus there appears a 

 mass of abnormal material called the " cellular inclusion." 



This mass was for long regarded as a parasite, as the visible 

 stage of a protozoon which possessed other stages more minute 

 or ultra - micro- 

 scopic. Nowadays 

 it is known that it 

 is merely a de- 

 formation of the 

 nucleus appearing 



after the invasion FIG. 61. Negri bodies in rabies; the hypothetical 

 of the cell by the microbe of rabies at different stages. (After 



virus. It is, as it 



were, the stigma of the presence of a virus within the cell, or 



even within the nucleus. 



These stigmata have been described under different aspects 

 and names in small-pox, vaccinia (Guarnieri bodies), rabies 

 (Negri bodies), molluscum contagiosum (described by Virchow), 

 trachoma, and the jaundice of the silk-worm, and the similarity 

 of these infections can no longer be doubted, 



