INFECTION AND IMMUNITY 351 



Certain microorganisms are invariable commensals 

 of the higher organisms, frequenting various surfaces 

 of the body upon which or in which the conditions of 

 life are favorable to them. When accident determines 

 that such shall be carried into the tissues, they may or 

 may not be able to survive the change. In most cases 

 they quickly succumb to the unusual conditions. In 

 other cases they survive -for a limited time, during 

 which the host suffers more or less disturbance, and 

 after which their vitality wanes, they die out and the 

 damage they have effected may be repaired. Far more 

 injurious are the new and strange microparasites with 

 which one occasionally comes into contact. Some 

 of these are actively invasive, find their way from the 

 skin, the respiratory or the alimentary organs into the 

 blood, distribute throughout the body, sometimes 

 exciting local histological changes in the tissues, some- 

 times general physiological disturbances, as fever, etc. 

 Such organisms may quickly or slowly destroy the host, 

 though in perhaps a majority of the cases they, too, 

 become less vigorous as time goes on, and die out, leav- 

 ing the patient to recover if not too much damaged by 

 their inroads. 



All grades of invasiveness occur. Some micro- 

 parasites find a very superficial and restricted field of 

 operation; some penetrate more deeply and are carried 

 in small numbers in the lymph and blood vessels to the 

 viscera where they slowly occasion minute changes, and 

 still others freely distribute through the blood and 

 affect the entire constitution of the host. 



The products of the microparasites must not be 

 neglected when considering their pathogenesis; they pass 

 through all grades of harmf ulness. Some microorganisms, 

 though they invade easily, produce only a mild fever; 

 others incapable of extensively invading the body 

 of the host, form poisonous products which when ab- 

 sorbed into the blood affect tissues remote from the seat 

 of microparasitic invasion. This is well exemplified by 



