INFECTION AND INTOXICATION. 63 



bacillus and streptococcus in the intestine and the staphy- 

 lococcus in the nose and mouth, infection takes place 

 when the bacteria invade the tissues. 



The mere entrance of bacteria into the tissues is not 

 sufficient to constitute infection, because should the 

 entering bacteria belong to varieties whose metabolic 

 products are without effect upon the tissues no evidence 

 of the invasion is apparent. Infection, therefore, implies 



(1) the invasion of the body or its tissues by bacteria and 



(2) injury of the body by the bacteria. 



Our knowledge has not yet progressed sufficiently to 

 permit us to say that any known bacterium may not, 

 under certain appropriate conditions, be a cause of disease, 

 even though there are many species, such as the Bacillus 

 subtilis and Bacillus prodigiosus, which have never been 

 known to do so. In speaking of bacteria we constantly 

 speak of pathogenic and. non-pathogenic species, as if there 

 were some fixed difference by which we could separate 

 them; yet a bacterium harmless for one animal is danger- 

 ous for another, and one usually incapable of harming an 

 animal may work havoc in its tissues should certain 

 unusual and abnormal conditions present themselves. 



One of the best illustrations of disease produced by 

 (usually) harmless bacteria is the condition known as 

 sapremia. In it we find that the growth of saprophytic 

 bacteria in the discharges from wounds, and in gan- 

 grenous areas, leads to the formation of toxic ptomains, 

 which, being absorbed by the lymphatics and into the 

 blood, produce fever and other disagreeable symptoms. 

 In this sapremic condition it is the activity of the bacteria 

 in the dead or effete matter, not their invasion of the 

 sound tissue, that causes the trouble, and the condition 

 may depend upon the presence of bacteria that are entirely 

 unable to live in the tissues of the body. 



The ability of bacteria to live in the tissues of a living 

 animal varies very greatly. Thus, there are purely sapro- 

 phytic bacteria that cannot live in the tissues at all; occa- 

 sionally parasitic bacteria that usually enjoy a saprophytic 



