596 INFECTION AND IMMUNITY. 



on the extent to which tuberculosis develops among 

 Scandinavians after they come to this country. 

 ; <Hereditary Nothing is commoner than the occurrence of 

 acy ' ' several successive cases of phthisis in the members 

 of a family, and the expression, heard on all sides, 

 that "tuberculosis is in the family," indicates the 

 general belief that a family tendency may be trans- 

 mitted from generation to generation. During 

 recent years, however, closer analysis of the condi- 

 tions has led many to doubt the existence or, at any 

 rate, the importance of family tendency or inher- 

 ited predisposition, and to refer the frequent oc- 

 currence of tuberculosis in a family to the greater 

 exposure to infection which is occasioned by close 

 contact with a pre-existing case. Cornet, who has 

 made a close statistical study of tuberculosis, dis- 

 credits entirely the hypothesis of hereditary pre- 

 disposition, and Cornet and Meyer refer to the 

 "habitus plithisicus" which we are disposed to 

 look on as an objective evidence of hereditary ten- 

 dency, as a result rather than a cause of pulmonary 

 tuberculosis. It is fair to say that the development 

 of tuberculosis in several members of a family is 

 not prima facie evidence of the existence of a 

 family predisposition for the disease. Where thera 

 are tubercle bacilli there is likely to be tuberculosis, 

 and the occurrence of the infection in one fur- 

 nishes the prerequisite, that is, bacilli, for the de- 

 velopment of the disease in other members of the 

 family. It is probable that the verdict of family 

 tendency has often been pronounced erroneously. 

 At present, however, we may not be justified in 

 considering the subject a closed chapter. 



It is the commonly accepted opinion that recov- 

 ery from tuberculosis does not confer immunity to 



