KARMKRS' IXSTriTTKS. 81") 



mouth and joint disfasi's <H,Hiir in vi'iy yoiini; pijis. cliok'ia |iiiii(ii)ally 

 ainoiisr the youiijr. sAviiie iila,iciu' aiuoiii; jtij;s and tlioso tliat arc half grttwn. 

 while swine plague attacks the old. half grown and older animals. It is 

 not meant to convey the ijupression that tlu'se diseases may not oeeiir at 

 any age liut that tlie l.irge |)rei>onderance of all cases do occtir at the time 

 indicated and that there is a ditterence in resistive power at different ages 

 independent of the effect of previous attacks. 



Kft'ect of Previous Attack.— Immunity results from an atttick of some 

 diseases. We can divide diseases into two classes upon this basis, tho.<?e 

 which do not tend to occur after one attack and those in which the 

 tendency is but little or not at all diminished. In the diseases of the fii*.st 

 class there are changes Avhich take place in the l)ody that protect it for a 

 greater or less length of time aaid in some as long as the subject lives. In 

 the second class these changes are so slight that they have only a tempo- 

 rary, if any, effect. The immunity thus acquired is not necessarily due to 

 changes caused bj' the presence of the disease germs but maj' be due to 

 the products which they produce. The products of the di-sease germs are 

 called toxines. That the immunity which results from an attack of some 

 disease is due to these poisons and not to the presence of the germs can be 

 easil.v demonstrated. The germs ma.v be grown in a suitable medium, as 

 bouillon, and after they have grown for some time the material may be 

 liltered and heated so that there Avill l)e no living germs present. This 

 material will contain the toxiue or poison produced by the germ and if 

 injected into the bod.v of a snsceptiide patient, first in small, and tlien 

 gradually increasing doses it will be found that the patient will acquire 

 immunity the same as that following an attack in the usual way. In this 

 instance there has been no germs, and yet immunity results. The body has 

 formed the same protective i)roducts to neutralize the poison introduced 

 artificially as would have occurred if they had resultefl from the growth 

 of the germs in the bod.v. 



In the second class of diseases, there are no protective products formed, 

 or if thej" are they are too Aveak to be effective. 



It must also be borne in mind that different diseases attack the body 

 in different ways and that the protective powers also differ. Somt> i)roduce 

 liieir eft'ects through a poison or toxine as already cited and the resistance 

 c<»mes from the autitoxine formed. The two Itest kn<»wn and most thor- 

 oughly studied diseases of this tyi)e are dii)htheria and tetanus or lock- 

 jaw. Both the toxines and atititoxines are so well known that the latter 

 is extensively \ised in tlie protection and cure, especially for diphtheria. 

 Some disi'ases affect the liody through the enormous multiplication of the 

 germs. In this t.vpe there niay be a product formed which tends to arrest 

 this multiplication or there ma.v be a special destructive activity on the 

 jiart of certain cells especially the white blood corpuscles. The product 

 which t«'nds to prevent multiplication is known as a bacteriolysim. Bac- 

 teriolytic jtruducts are not s<» well l<nown as aiilitoxines and not so sue- 



