FACTORS OF PATHOGENICITY AND INFECTION 183 



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 organ- 

 isms, although infectious when gaining entrance to tissues not abun- 

 dantly 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 themselves 

 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 viru- 

 lence than when freshly isolated from the bodies of man or ani- 

 mals. It is necessary, therefore, in order to produce infection, that 

 the particular bacterium involved shaU 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 introduction 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 entrance 

 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 strepto- 

 cocci, when swallowed, may cause no harmful effects, while the same 

 bacteria rubbed into the skin may give rise to a severe reaction. 



Animals and man are protected against invasion by bacteria in 

 various ways. Externally the body is guarded by its coverings of skin 

 and mucous membranes. When these are healthy and undisturbed, 

 microorganisms are usually held at bay. While this is true in a gen- 

 eral way bicteria may in occasional cases pass through uninjured 



