272 THE INFECTIOUS DISEASES. 



degeneration. The nature of the infective agent in typhus is unknown. 

 Several observers have recorded the finding of micro -organisms of one 

 kind or another in the body during life and after death, but proof that 

 any of these are excitants of the disease has not yet been furnished. 



YELLOW FEVER. 



LESIONS OF THE DISEASE. 



This infectious disease is without characteristic lesions save for the 

 haemorrhages and pigmentation in the skin. Such other lesions as com- 

 monly exist are those common to toxaemia. The following conditions 

 are, however, frequently present after death : 



Eigor mortis is marked and occurs early. 



The brain and its meninges are usually congested. The skin is of a 

 yellow color from the presence of bile pigment, and may be mottled by 

 ecchymoses. 



The heart is of a pale or brownish-yellow color. Its muscular fibres 

 may be the seat of fatty degeneration. The lungs may be congested. 



The Stomach often contains a characteristic dark fluid, due to altered 

 blood pigment, similar to that which is vomited during life black vomit. 

 Its mucous membrane may be congested, softened, and is sometimes 

 eroded. The intestines are dark-colored, often distended with gas, and 

 sometimes contain blood. The liver in the earlier stages of the disease 

 may be intensely congested. More frequently it contains but little blood, 

 is of a light-yellow color, and the hepatic cells show the changes of an in- 

 tense albuminous degeneration, often much more marked than are found 

 in any other disease except acute yellow atrophy of the liver. Areas of 

 focal necrosis may be present. The gall bladder is apt to be contracted. 



The spleen shows no marked changes. The kidneys present an in- 

 tense albuminous degeneration. The tubules usually contain masses of 

 hyalin material. 



THE EXCITANT OF THE DISEASE. 



While its mode of occurrence and the characters of its symptoms and 

 lesions afford a strong presumption that yellow fever is an acute infec- 

 tious disease, none of the various studies which have been made upon its 

 etiology have as yet revealed the presence of any micro-organism which 

 can be confidently accepted as its excitant. ' 



THE MODE OF INFECTION. 



It has been shown by Eeed and his colleagues, Carroll and Agramonte, 

 that the infectious agent in yellow fever may be transmitted by the sub- 



1 The various studies of Sternberg, who isolated a bacillus which he called " Bacillus 

 x," and of Sanarelli, who found a bacillus which he named Bacillus icteroides, are the 

 most noteworthy earlier contributions to the subject. A later study and references to 

 the bibliography of this subject may be found in an article by Heed and Carroll, Jour. 

 Exp. Med., vol. v., p. 215, 1900. 



