8 VACCINE AND SERUM THERAPY. 



Besides conditions in the causal organism, certain conditions 

 must exist in the host or body so that infection can occur. Cer- 

 tain animals are insusceptible to the action of certain species of 

 microorganism, thus for example, the horse and other domestic 

 animals are naturally immune to venereal diseases. Again cer- 

 tain organisms can only produce disease when they are present 

 in certain parts of the body and have entered the body by certain 

 portals of entry. The spirillum cholerae can only produce its 

 typical form of disease when it has gained entrance through the 

 small intestine. Invasion of the dermis or epidermis by this or- 

 ganism produces no disease. The bacilli producing tuberculosis as 

 well as certain other organisms, can cause infection when they 

 have entered through the skin or through the mucous membrane. 

 Certain organisms as Bact. diphtheriae, B. tuberculosis, and B. 

 typhosus, show predilections for certain tissues but will also pro- 

 duce diseased conditions in other parts of the body. 



In addition to the non-susceptibility of the host, the body 

 possesses certain natural barriers to disease which must be over- 

 come before infections can be produced. The unbroken skin 

 usually offers a barrier to infection. Age, sex, race, occupation, 

 etc. , at times account for certain resistance to invasions of organ- 

 isms. The resistance furthermore varies for the different organ- 

 isms, as is evidenced by the fact that for B . tetanus a wound 

 is necessary to produce lock-jaw, while for the glanders bacillus 

 the slightest abrasion of the muccus membrane will furnish a 

 focus for an infection. The body fluids, lymph glands, phagocytes, 

 all offer resistance to infection. These barriers are overcome in 

 various ways and under different conditions, so that while indi- 

 viduals may be immune at one time they may, at another time, 

 be susceptible to the same infectious agent. 



COURSE OF INFECTIONS. 



From the foregoing it is evident that not only must the in- 

 fectious organism come into contact with the body tissues to 

 produce disease, but the organism must be able to grow, multi- 

 ply, produce its poison, and overcome the resistance of a body 

 susceptible to its action. The symptoms and signs of infections 

 do not appear until a certain time after the invasion by the micro- 



