34 
Tuberculosis. 
contagious  disease  it  is  obvious  that  this  fact  may  admit  of 
another  explanation,  viz.,  that  exceptional  risks  of  infection, 
and  not  exceptional  susceptibility  transmitted  by  parent  to 
offspring,  may  be  at  the  root  of  family  tuberculosis.  In  the 
case  of  human  beings,  tuberculous  parents,  in  the  immense 
majority  of  cases,  inhabit  the  same  houses  and  even  the  same 
rooms  as  their  children,  and  hence  as  a rule  such  children 
are  exposed  to  a great  risk,  against  which  until  recently 
no  precautions  whatever  were  taken.  When  full  weight  is 
allowed  to  this  consideration  there  does  not  appear  to  be  any 
reason  to  assume  the  existence  of  a special  predisposition  to 
account  for  the  fact  that  human  tuberculosis  tends  to  run  in 
families. 
The  truth  is  that  importance  would  never  have  been 
assigned  to  inheritance  of  a special  predisposition  if  it  had 
been  recognised  from  the  first  that  tuberculosis  is  a conta- 
gious disease.  It  was  a reasonable  explanation  of  the  observed 
facts  in  connection  with  human  consumption  before  it  had 
been  proved  that  no  case  of  it  can  arise  in  any  family  except 
through  infection  with  bacilli  derived  from  some  previous 
subject  of  the  disease  ; but  as  soon  as  that  fact  was  recognised 
it  became  unreasonable  to  assume  that  inherited  special 
predisposition  was  the  principal  cause  of  the  occurrence  of 
successive  cases  in  the  same  family. 
At  the  present  day,  therefore,  it  must  be  denied  that  the 
existence  of  family  predisposition  is  proved  by  the  fact  that 
cases  of  tuberculosis  are  more  frequent  among  the  children 
of  tuberculous  parents  than  among  those  whose  parents  are 
healthy.  An  attempt,  however,  is  sometimes  made  to  reconcile 
belief  in  the  importance  of  inherited  special  predisposition 
with  the  fact  that  tuberculosis  is  a contagious  disease,  (T)  by 
assuming  that  in  civilised  communities  the  tubercle  bacillus  is 
so  widely  distributed  that  every  one  is  constantly  or  frequently 
exposed  to  risk  of  infection,  and  (2)  by  citing  cases  in  which 
various  members  of  the  same  family  have  in  succession 
developed  tuberculosis  long  after  separation  from  the  diseased 
parent  and  from  each  other. 
But  the  assumption  that  the  tubercle  bacillus  is  ubiquitous 
is  opposed  to  a great  mass  of  experimental  evidence.  It  is 
true  that  since  no  restrictions  are  placed  on  the  movements 
of  tuberculous  human  beings  no  one  can  escape  all  risk  of 
infection,  but  the  average  risk  attributable  to  this  circumstance 
is  nothing  like  so  great  as  the  risk  to  which  those  are  sub- 
jected who  from  the  day  of  their  birth  live  in  close  association 
with  their  already  diseased  parents. 
As  to  the  late  development  of  tuberculosis  by  persons  long 
after  their  removal  from  what  may  be  called  the  consumptive 
