72 Causes of Disease. 



upon the tissues of the body depends in part upon their metaboUc 

 activity and metabohc products. From investigations of Wiener 

 it has been shown conclusively that a type of bacteria common to 

 the human intestine and regarded as entirely harmless in this 

 situation and in the excreta, can by special nutritive influence be 

 so transformed as to acquire highly toxic properties (the so-called 

 eolon bacilli, living as saprophytes in the colon). If fed ordina- 

 rily to rats they give rise as a rule to no disturbances whatever, 

 but if the colon bacilli have been cultivated for a few days under 

 anaerobic conditions on a medium specially rich in albumen (eggs) 

 they acquire an intense virulence, and if fed to rats produce a 

 severe ami almost invariably fatal enteritis. Although such 

 transformation of a non-toxic saprophyte into a toxic pathogen 

 cannot ofif-hand be accomplished with other micro-organisms, this 

 isolated fact proves that it is possible for infections of a character 

 new and imknown to us to arise from time to time ; and as a 

 matter of fact spontaneous infectious diseases do now and again 

 appear which have not been previoi:sly observed and which are 

 to be regarded as rarities. It is a pure accident when the special 

 germs are brought into contact with the human and animal body ; 

 and it is further conditional upon special circumstances of nutri- 

 tion whether the microbe possess virulent properties, as in case 

 of the meat intoxications caused by various microorganisms, 

 sausage poisoning, diarrhoea and vomiting and wound infections. 

 Some infectious diseases are of very frequent occurrence because 

 the agent is widel}- disseminated in nature or because opportunities 

 for transmission to the animal body are often afforded. For in- 

 stance, the tetanus bacillus exists in the soil in many localities, 

 and wounds are often contaminated by dirt, the opportunitv for 

 tetanus infection being correspondingly common. Such wide- 

 spread microorganisms and their infectious diseases are spoken 

 of as ubiquitous. As the alpine rose and reindeer moss, palms 

 and cacti flourish only within certain geographical limits, and as 

 poisonous snakes and the different kinds of insects have their 

 habitats only in certain parts of the earth ; so certain microbes 

 occur only in certain lands, where alone, too, the corresponding 

 infectious diseases arise as spontaneous affections. Such districts 

 are foci in which annually a certain number of cases are likely 

 to occur, and it is said that the disease is established in such 

 places {indigenous, cudeuiic). Truly contagious diseases ma}- 

 also be endemic, provided in a certain district (irrespective of soil 



