CHAPTER III. 



INFECTION. 



/ Infection means the successful invasion of the tissues of the body 

 by either animal (protozoa, vermes) or vegetable (bacteria and 

 moulds) organisms with the evidences of their action. To success- 

 fully infect the body, bacteria must enter the tissues, be of sufficient 

 number, find the tissues receptive, and continue to multiply. 



The skin, mucous membranes, and the various cavities of the body 

 connected with the outside air, teem with countless bacteria at all 

 times, many of which are pathogenic, yet there is no infection, be- 

 cause the tissues are not invaded. Again, there can be no doubt 

 that highly pathogenic bacteria enter the tissues of healthy people 

 at times, in small numbers, and yet no disease is produced, because 

 of their scarcity, or by reason of the tissues not being receptive. 

 Infection implies not only invasion of the body, but injury to the 

 tissue. Certain bacteria may invade a body, and yet create no 

 harm. These bacteria may enter dead or dying body tissues, and 

 secrete poisonous substances (toxins) which may be absorbed, and 

 produce pathologic symptoms known as Sapr&mia. Clots of 

 blood in the parturient uterus, and gangrenous limbs may be in- 

 vaded by strict saprophytes incapable of life in living tissues, and 

 yet cause much harm by the absorption of their products. 



Infestation is when bacteria, even pathogenic, are present in a 

 place without exciting a reaction. Matter carrying pathogenic 

 germs is called infective, 



Depending upon their ability to grow in the body, bacteria may 

 be divided into (i) purely saprophitic; (2) occasionally parasitic; 

 and (3) purely parasitic. A host harbors a parasite. 



Purely saprophitic germs cannot live in tissues at all; those that 



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