74 Infection 



actually grow through the capillary walls, or that they reach 

 the blood circulation 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. 



1. Virulence. Virulence may be defined as the disease- 

 producing power of micro-organisms. It depends upon the 

 invasiveness of the bacteria or the toxicity of their products. 



It is a variable quality of bacteria as they are found in the 

 morbid conditions with which they are associated. It is 

 a still more variable quality of most bacteria as they are 

 cultivated in the laboratory. A few bacteria are almost 

 constant in their pathogenic power, and cultures of 

 Bacillus tuberculosis and B. mallei can be kept under arti- 

 ficial conditions for years with very little change. Some 

 other bacteria begin to diminish in activity so soon as they 

 are introduced to the artificial conditions of life in the test- 

 tube, and in a few days, or weeks or months, as the case may 

 be, lose their pathogenic power altogether or in large part. 

 Still others, and perhaps the greater number, can be modified, 

 and their pathogenic activity increased or diminished accord- 

 ing to the experimental manipulations to which they are 

 subjected. 



Variation in virulence is not always a peculiarity of the 

 species, for the greatest difference may be observed among 

 individuals of the same kind. Thus the streptococcus 

 usually attenuates rapidly when kept in artificial media, and 

 various measures have been suggested for modifying and 

 maintaining it, but Hoist observed one culture whose viru- 

 lence was unaltered after eight years' cultivation in the 

 laboratory without any particular attention having been 

 devoted to it. 



Decrease of virulence under artificial conditions prob- 

 ably depends upon artificial selection of the organisms in 

 transplantation from culture to culture. When planted 

 upon artificial media, the vegetative members of the bacte- 

 rial family proceed to grow actively and soon exceed in 



