ETIOLOGY. 23 



through the tissues, when the germ is multiplying in the animal 

 body. In at least some species the formation of a toxin is not a 

 phenomenon which invariably accompanies growth and multipli- 

 cation. This is shown to be true by the frequently observed fact 

 that a highly virulent germ may under certain conditions wholly 

 lose its toxicity while it continues to vegetate most luxuriantly. It ■ 

 seems to be evident that certain conditions of growth, as, for in- 

 stance, the nature of the medium, the temperature, the supply of 

 oxygen, and the presence or absence of certain chemical ^ents, de- 

 termine the amount of specific toxin formed within the cell. Most 

 pathogenic germs find the conditions suitable for the elaboration 

 of poisons at their optimum in the animal body, and for this reason 

 their virulence is increased by passing successively through a series 

 of animals. However, this is not always true, and germs may de- 

 crease in virulence by being passed through certain animals ; some 

 observers have reported the finding of certain microorganisms that 

 invariably decrease in virulence on being transferred from one 

 animal to another. There are many interesting questions along this 

 line which need to be investigated much more fully than has been 

 done up to the present time before we can speak positively concern- 

 ing them. They furnish the basis of problems that are of both 

 scientific and practical interest. In many epidemics the specific 

 microorganism, to which the epidemic disease is due, apparently in- 

 creases for a while in virulence and then gradually seems to become 

 less dangerous. However, a discussion of these questions would take 

 us too far away from the subject matter just now in hand. 



We will now give what appears to us, in the present state of our 

 knowledge, a correct definition of an infectious disease : 



An inifectious disease arises when a specific, pathogenic microor- 

 ganism, having gained admittance to the body and having found con- 

 ditions favorable, grows and multiplies, and in so doing elaborates a 

 chemical poison which induces its characteristic eflFects. 



In the systemic infectious disease, such as anthrax, typhoid fever 

 and cholera, the specific poison is undoubtedly taken into the gen- 

 eral circulation, and may reach and influence every part of the body. 

 In the local infectious diseases, such as gonorrhea and infectious 

 ophthalmia, the first action of the poison seems to be confined to th^ 

 place of its formation ; although even in these, when of a specially 

 virulent type, the effects may extend to the general health, or the 

 poison may strongly act on some distant part of the body. It is 

 probably true that in many of the infectious diseases the chemical 

 poison has both a local and a systemic action ; thus, it is by no means 

 certain that the ulceration of typhoid fever is due directly to the 

 living bacillus, for it is now an established fact that this disease may 

 exist, run a typical course, and end in death, without anatomical 

 changes in the intestine. In diphtheria and tetanus the toxin 



