House-flies  are  often  carriers  of  disease  germs  which  adhere
to  their  bodies  when  the  flies  crawl  over  filth.  Flies  are  acci-
dental carriers  of  disease  germs.  They  would  not  have  disease
germs  on  their  bodies  if  human  beings  had  no  garbage  heai>s  or
foul  collections  of  excretions  on  which  the  flies  can  crawl.  House-
flies  do  not  bite  or  scratch,  and  so  they  do  not  introduce  the  germs
directly  into  the  body,  but  they  leave  them  on  food  on  which
they  alight,  and  on  the  mouths  and  eyes  of  babies.  The  prin-
cipal fly-borne  diseases  are  typhoid  fever,  infantile  diarrhea,  and
infections  of  the  eyes.
Air-borne  Diseases. — It  was  formerly  thought  that  nearly
every  communicable  disease  was  air-borne;  but  experience  and
scientific  investigations  prove  that  the  air  is  almost  entirely
free  from  disease  germs,  except  the  foul  air  of  very  close  and
dirty  rooms.  Even  in  foul  air  the  disease  germs  are  usually
associated  with  gross  particles  of  matter  which  may  be  seen.
Most  infective  particles  in  the  air  consist  of  moist  droplets  of
mucus  and  saliva,  but  dry  dust  may  also  contain  living  bacteria
of  disease  under  favorable  conditions.
Disease  germs  usually  die  when  they  are  dried,  but  they  may
remain  alive  for  a  few  minutes  or  hours.  When  a  crowd  gathers
in  a  close  room,  some  diseased  person  is  likely  to  spit  on  the
floor.  The  sputum  may  quickly  dry,  and  its  dust  may  be  scat-
tered through  the  air  before  the  disease  germs  have  time  to  die.
