ACUTE INFECTIOUS DISEASES. 



morbific germ of intermittent depends. In the same way, experience 

 shows that the number of typhus cases, in any place where the disease 

 prevails, varies with the greater or less moisture of the soil ; this is 

 simply because certain degrees of moisture of the ground are favorable 

 or unfavorable to the decomposition of animal substances on which 

 the production of the typhus poison depends. It has often been ob- 

 served that the sudden drying of previously moist soil, or, as is usually 

 said with doubtful propriety, the sudden fall of the sub-soil water, cor- 

 responded with an increase of the typhus cases. The explanation of 

 this is simple, according to what we have already said. But even the 

 exceptions, where a moist state of the previously dry soil, sudden rise 

 of the sub-soil water, coincided with an increase of the disease, are not 

 surprising, nor are those where, while the height of the water remains 

 the same, the number of cases varies. Of course other causes besides 

 the moisture of the soil influence the decomposition of animal sub- 

 stances, and consequently affect the production of typhus germs. 



The susceptibility to typhus poison varies greatly with the individ- 

 ual. There is one very interesting point, which is also seen in other 

 infectious diseases, that persons who have lived for some tune in a 

 place peculiarly liable to the disease (as Munich), without being at- 

 tacked, are in less danger when epidemics occur than those who have 

 recently come to the place. It can scarcely be supposed that this is 

 because the former were originally less disposed to the disease than the 

 latter, but because, after a long absence from their native place, on 

 their return to it, they are just as liable to the disease as new-comers 

 who have only been there a short time. This fact probably depends 

 on some unexplained accommodation to typhus poison. Statistics 

 concerning the influence of age, sex, condition of life, and constitution, 

 on the susceptibility for typhus, have shown that infants and old people 

 are rarely attacked by abdominal typhus ; persons of middle age are 

 most liable to it ; males are attacked oftener than females ; strong, 

 well-nourished persons, oftener than weak, badly-nourished ones ; and 

 that the disease is proportionately more frequent among the poor than 

 among the well-to-do classes. Formerly, it was generally believed 

 that tuberculous patients were never attacked by abdominal typhus ; 

 this is not absolutely true, although it rarely occurs. The same is 

 true of patients with heart-disease, carcinoma, and other chronic or 

 acute diseases, and of pregnant or nursing women. Pregnancy gives 

 almost absolute immunity to the disease. Except in rare cases, one 

 attack removes the susceptibility. Since 1820-1830, during which 

 time exanthematic typhus has become more rare, abdominal typhus 

 has become more frequent. With the exceptions mentioned in the pre- 

 ceding chapter, it is the common form of typhus throughout Europe. 



