Jiifcctioiis .li^ciits. 69 



marked variations in the clinical and anatomical picture of 

 the same infectious disease. I'hus a certain strain of glanders 

 bacilli has been known to cause in a horse a chronic glanders 

 lasting over several years, but in guinea-pigs and field mice a 

 very acute type of the disease; the bacillus of hog-erysipelas, pro- 

 ducing in one hog an acute septicaemia, to cause in a second a 

 slight urticaria, in a third a chronic cardiac valvular affection. 

 The influence of tissue resistance upon the disease picture is 

 especially apparent when inoculations of the organisms of chicken- 

 cholera of high grade of virulence are made into rabbits previously 

 treated by serum injection. Rabbits which have not been previ- 

 ously subjected to serum injection succumb to hypodermic inocu- 

 lation of but a small number of bacteria as early as from 12 to 24 

 hours, dying of severe acute septicaemia ; those previously injected 

 and then subjected to the same inoculation die only after 5 to 14 

 days, and then show extensive purulent phlegmons, purulent and 

 fibrinous pleurisy and pericarditis, or merely an ansemia as a re- 

 sult of the prolonged infection. 



Recovery from an infectious disease usually leaves thereafter 

 a certain degree of ininiunity (v. active immunity, p. 2/). 



Sometimes several kinds of pathogenic organisms gain en- 

 trance at the same time through one point of infection, the result- 

 ant infection being thus a complicated one (mixed infection) ; or 

 after the individual is attacked by one infection other types may 

 invade (secondary mixed infection). The former condition is 

 seen when the infective matter from the beginning contained sev- 

 eral varieties of pathogenic germs, as where a wound is contami- 

 nated with earth containing both spores of tetanus bacilli and pus 

 bacteria, or where a cow's nipple is soiled with filth and dung 

 m which there are streptococci and colon bacilli. So, too, micro- 

 organisms which have existed indefinitely upon the surface of 

 some mucous membrane may, if occasion presents, penetrate into 

 the tissues along with some other type of infection and combine with 

 the latter in producing pathogenic effects ; thus, should there be pro- 

 duced by some foreign body an injury to the tongue or pharynx 

 of the cow, the various putrefactive organisms living as sapro- 

 phytes in the mouth may occasion with pyogenic bacteria a mixed 

 infection. The second form of mixed infection, the successive 

 invasion of a second or third pathogenic germ, may be noted 

 where local changes 'from a primary infection bring about condi- 

 tions which facilitate the access and growth of other microbes; as 



