The Cardinal Conditions of Infection 79 



foothold in diseased tissues and assist in morbid changes. This 

 is seen in what is described as sapremia, in which various sapro- 

 phytic bacteria, possessing no invasive powers, by growing in the 

 putrefying tissues of a gangrenous part, give rise to poisonous sub- 

 stances which when absorbed by the adjacent healthy tissues 

 produce such constitutional disturbances as depression, fever, and 

 the like. 



Bacteria with limited invasive powers and intracellular toxins 

 can at best occasion local effects. Such organisms not infrequently 

 vary, however, and when of unusual vitality may survive entrance 

 into the blood and lymph circulations and occasion bacteremia, or, 

 as it is more frequently called, septicemia, a morbid condition 

 characterized by the presence of bacteria in the circulating blood. 

 When bacteria entering the circulation are unable to pervade the 

 entire organisms, they may collect in the capillaries of the less re- 

 sisting tissues, producing local metastatic lesions, usually purulent 

 in character. This results in what is surgically known as pyemia. 



The mode by which the entrance of bacteria into the circulation 

 is effected differs in different cases. Kruse* believes that they some- 

 times are passively forced through the stomata of the vessels when 

 the pressure of the inflammatory exudate is greater than that of 

 the blood within them; that they may sometimes enter into the bodies 

 of leukocytes that have incorporated them; that they may actually 

 grow through the capillary walls, or that they reach the blood cir- 

 culation indirectly by first following the course of the lymphatics. 



Toxemia results from the absorption of the poisonous bacterial 

 products from non-invasive bacteria, as in tetanus. 



THE CARDINAL CONDITIONS OF INFECTION 



Infection can take place only when the micro-organisms are 

 sufficiently virulent, when they enter in sufficient number, when 

 they enter by appropriate avenues, and when the host is susceptible 

 to their action. 



Virulence. — Virulence may be defined as the disease-producing 

 power of micro-organisms. It is a variable quality, and depends 

 upon the invasiveness of the micro-organisms, or the toxicity of their 

 products, or both. 



A few bacteria are almost constant in virulence and can be kept 

 under artificial conditions for years with very little change. Other 

 bacteria begin to diminish in virulence so soon as they are introduced 

 to the artfficial conditions of life in the test-tube. Still others, and 

 perhaps the greater number, can be modified, and their virulence 

 increased or diminished according to the experimental manipulations 

 to which they are subjected. 



Variation in virulence is not always a peculiarity of the species, 

 * Flugge, "Die Mikroorganismen," vol. I, p. 271. 



