CARRIERS  AND  MISSED  CASES  69

Shiga  1  says  that  the  bacilli  generally  remain  1  or  2  weeks,
and  that  perfectly  normal  persons  sometimes  harbor  the
germs.  Aveline,  Boycott  and  McDonald 2  failed  to  find  the
bacillus  in  27  contacts  in  an  asylum.
Cameron  3  also,  and  Macalister,4  rarely  found  bacilli  in
perfectly  healthy  contacts.  The  latter  writer  found  that
26  per  cent  of  the  cases  studied  by  him  in  a  certain  asy-
lum developed  a  mild  chronic  state  or  had  relapses,  and
that  these  conditions  only  could  be  regarded  as  dangerous.
These  he  considers  as  practically  carriers,  and  says  their
importance  cannot  be  overestimated.
O.  Mayer  5  has  reported  rinding  entirely  healthy  carriers,
and  one  of  these  later  became  sick.  The  bacillus  found
was  of  the  "  pseudo-  "  type  F.  In  convalescents  he  found
them  persisting  up  to  202  days.  The  average  persistence
is  3  to  7  months.  In  healthy  persons  they  were  not  found
over  30  days.  Intermittency  of  excretion  was  noted  in
both  convalescent  and  healthy  carriers.
Chronic  Plague  in  Rats.  —  Bubonic  plague  is  a  disease
which  attacks  not  man  alone  but  many  other  species  of
animals,  particularly  the  rat.  In  fact  it  may  be  considered
primarily  a  rat  disease,  and  without  doubt  the  rat  is  the
most  important  agent  in  its  diffusion.  That  mild  cases
and  chronic  cases  exist  among  rats  which  superficially  ap-
pear not  to  be  sick,  seems  to  be  proved.  Simpson 6  says
chronic  plague  was  observed  in  some  of  the  animals  experi-
mented on  in  Hong  Kong,  and  by  Albrecht  and  Ghon  in
guinea  pigs,  and  in  rats  for  months  by  Kolle  and  Martini.
The  Indian  Plague  Commission  (1905)7  found  eleven  of  the

1  Shiga,  Philippine  J.  Sc.,  Manila,  1906,  I,  485.
