PAPERS OX MEDICINE AND PUBLIC HEALTH 463 



It is interesting to know that approximately only 50% 

 of children of school age are susceptible to diphtheria. 

 In other words, about one-half of the children are natur- 

 ally immune to this disease. They would not get the 

 disease even upon the closest contact with a case of diph- 

 theria. This is due to the fact that there exists in the 

 blood of such children at least 1 30 of a unit of diiDh- 

 theritic antitoxin to each cc. of blood. The amount of 

 antitoxin in a child's blood can be as accurately deter- 

 mined as the composition of the rocks or the interplane- 

 tary distances. This natural immunity is technically 

 called active immunity and is believed to be permanent ; 

 that is, if after the first year of life the child is immune, 

 he will always be immune. Carefully distinguish be- 

 tween this active immunity and what is called passive 

 immunity, because I shall use these terms later. Passive 

 immunity is the immunity occasioned by the transitory 

 presence in the blood of sufficient antitoxin to prevent 

 or cure diphtheria. This condition of immunity may be 

 brought about by the direct injection of diphtheritic 

 antitoxin into the child's blood or tissues. In practice, 

 1000 or 1500 units of antitoxin are injected into a child 

 who has been exposed to diphtheria to prevent his get- 

 ting it. The immunity so conferred is passive immunity. 

 It is short lived, lasting about four weeks, after which 

 the child is as susceptible as ever and perhaps more 

 susceptible. 



It will be conceded readily that it is important to de- 

 termine if possible -by some safe simple test a child's 

 status with reference to diphtheria. If he is permanent- 

 ly immune, he will not get diphtheria nor will he need 

 the immunizing dose of antitoxin if exposed to diph- 

 theria. Sore throat of some kind is one of the most fre- 

 quent diseases of childhood: comparatively few cases are 

 true diphtheria but every case is potentially diphtheria, 

 ience thousands of bottles of diphtheritic antitoxin are 

 needlessly given every year in this state. 



Until nine years ago the only way to determine 

 whether a child was susceptible or not was by a tedious 

 process of blood examination. In 1913, an Austrian 

 physician by the name of Schick devised a simple skin 



