Disease Transmission and Mans Resistance 325 



technic for treating Mycobacterium tuberculosis to make the 

 organisms avirulent. Vaccines made from dead bacteria fail to 

 provide protection against tuberculosis. B.C.G. vaccine is prepared 

 bv growing the tuberculosis organisms on special media at tem- 

 peratures above optimum for the bacteria. This attenuates their 

 virulence, and living Mycobacterium tuberculosis organisms can be 

 introduced as vaccines to stimulate active immunity to the disease. 



Extensive field tests have been underway in France for a 

 number of years, but it will require several generations before the 

 full interpretation of the results can be made. Those who are 

 working on the project, however, are encouraged by what they 

 have seen to date, and this has prompted public health authorities 

 in the United States to institute similar programs in areas where 

 tuberculosis rates are high. 



Since tuberculosis is a major health problem during periods 

 of war and famine, B.C.G. vaccine is being put to use in many 

 far-flung parts of the world. Again, it is too early to draw con- 

 clusions as to the merits of the program. 



Toxoids 



A Toxom is a toxin that has been chemically treated, usually 

 with formalin, to remove its toxic properties without interfering 

 with its antigenic qualities. Toxoids, therefore, can induce an 

 active immuntiy and serve a useful function in protecting in- 

 dividuals for varying periods of time. An active case of a disease 

 caused by toxins, however, must be treated with antitoxin to afford 

 immediate protection; this is passive immunity and does not endure 

 for more than a short time. Diphtheria, tetanus, and gas gangrene 

 are three diseases for which effective toxoids are available. 



Gamma Globulin 



Adults who have been vaccinated against certain diseases, or 

 who have actually had clinical symptoms of the disease, possess 

 antibodies which are found in the protein portion of the blood 

 known as globulin. Globulin, in turn, can be further broken 

 down into smaller fractions, one of which is called gamma 



