232 INFECTION AND IMMUNITY 



to live and multiply in the animal fluids and tissues. For instance, 

 an organism which is shown not 'to grow at the body temperature 

 of warm-blooded animals may safely be assumed not to be infectious 

 for such animals; and experience is gradually teaching us that 

 strictly aerobic organisms, those thriving only in the presence of 

 free oxygen and not able to obtain this gas in available combination 

 from carbohydrates, can also be safely excluded from the infectious 

 class. We have also learned that anaerobic organisms, although 

 infectious when gaining entrance to tissues not abundantly supplied 

 with blood, are practically unable to multiply in the blood stream 

 and give rise to generalized infection. 



The pathogenic microorganisms differ very much among them- 

 selves in the degree of their disease-inciting power. Such power is 

 known as virulence. Variations in virulence occur, not only among 

 different species of pathogenic bacteria, but may occur within the 

 same species. Pneumococci, for instance, which have been kept upon 

 artificial media or in other unfavorable environment for some time, 

 exhibit less virulence than when freshly isolated from the bodies of 

 man or animals. It is necessary, therefore, in order to produce infec- 

 tion, that the particular bacterium involved shall possess sufficient 

 virulence. 



Whether or not infection occurs depends also upon the number of 

 bacteria which gain entrance to the animal tissues. A small number 

 of bacteria, even though of proper species and of sufficient virulence, 

 may easily be overcome by the first onslaught of the defensive forces 

 of the body. Bacteria, therefore, must be in sufficient number to 

 overcome local defenses and to gain a definite foothold and carry 

 on their life processes, before they can give rise to an infection. 

 The more virulent the germ, other conditions being equal, the smaller 

 the number necessary for the production of disease. The introduc- 

 tion of a single individual of the anthrax species, it is claimed, is 

 often sufficient to cause fatal infection ; while forms less well adapted 

 to the parasitic mode of life will gain a foothold in the animal body 

 only after the introduction of large numbers. 



The Path of Infection. The portal by which bacteria gain en- 

 trance to the human body is of great importance in determining 

 whether or not disease shall occur. Typhoid bacilli rubbed into 

 the abraded skin may give rise to no reaction of importance, while 

 the same microorganism, if swallowed, may cause fatal infection. 

 Conversely, virulent streptococci, when swallowed, may cause no 



