﻿212 
  INTRODUCTION 
  TO 
  BOTANY 
  

  

  both 
  diseased 
  and 
  healthy 
  twigs, 
  the 
  knife 
  may 
  be 
  the 
  means 
  

   of 
  transferring 
  bacteria. 
  If 
  all 
  infected 
  parts 
  are 
  removed 
  and 
  

   burned, 
  and 
  if 
  the 
  knife 
  used 
  in 
  pruning 
  diseased 
  twigs 
  is 
  

   sterilized 
  before 
  being 
  used 
  in 
  pruning 
  healthy 
  plants, 
  the 
  

   continued 
  spread 
  of 
  the 
  disease 
  is 
  made 
  unlikely. 
  

  

  197. 
  Diseases 
  of 
  man. 
  In 
  the 
  section 
  on 
  nutrition 
  of 
  the 
  

   bacteria 
  (sect. 
  188) 
  attention 
  was 
  directed 
  to 
  the 
  fact 
  that 
  ex- 
  

   cretions 
  are 
  regularly 
  produced 
  by 
  them. 
  In 
  case 
  of 
  disease- 
  

   producing 
  forms, 
  some 
  of 
  these 
  excretions 
  are 
  injurious 
  or 
  

   poisonous, 
  and 
  are 
  known 
  as 
  toxins. 
  In 
  susceptible 
  plants 
  or 
  

   animals 
  toxins 
  may 
  produce 
  disease. 
  Each 
  kind 
  of 
  disease- 
  

   producing 
  bacterium 
  forms 
  its 
  own 
  peculiar 
  toxin 
  or 
  toxins, 
  

   which 
  in 
  time 
  may 
  produce 
  a 
  particular 
  kind 
  of 
  disease. 
  Sub- 
  

   stances 
  that 
  neutralize 
  toxins 
  are 
  known 
  as 
  antitoxins. 
  In 
  the 
  

   body 
  of 
  the 
  diseased 
  organism 
  antitoxins 
  are 
  produced 
  which, 
  

   when 
  formed 
  in 
  sufficient 
  quantities, 
  counteract 
  the 
  influence 
  

   of 
  the 
  toxins. 
  When 
  one 
  has 
  had 
  an 
  attack 
  of 
  smallpox 
  or 
  

   diphtheria 
  and 
  has 
  produced 
  sufficient 
  antitoxin 
  to 
  enable 
  him 
  

   to 
  overcome 
  it, 
  he 
  is 
  usually 
  not 
  susceptible 
  to 
  another 
  attack 
  

   of 
  the 
  same 
  disease. 
  He 
  is 
  immune. 
  There 
  are 
  other 
  diseases 
  

   (such 
  as 
  mumps, 
  measles, 
  and 
  scarlet 
  fever) 
  against 
  which 
  

   most 
  people 
  may 
  acquire 
  immunity 
  by 
  once 
  surviving 
  an 
  at- 
  

   tack. 
  This 
  immunity 
  is 
  usually 
  lifelong, 
  though 
  exceptions 
  

   are 
  known. 
  In 
  the 
  case 
  of 
  other 
  diseases 
  (such 
  as 
  typhoid 
  and 
  

   influenza), 
  one 
  may 
  become 
  susceptible 
  to 
  another 
  attack. 
  

   Some 
  people 
  are 
  naturally 
  immune 
  to 
  certain 
  diseases. 
  

  

  Smallpox 
  vaccination 
  1 
  consists 
  in 
  infecting 
  human 
  beings 
  

   with 
  organisms 
  that 
  have 
  been 
  grown 
  in 
  such 
  unfavorable 
  

   ways 
  that 
  their 
  ability 
  to 
  produce 
  disease 
  is 
  greatly 
  reduced. 
  

   Consequently 
  the 
  result 
  of 
  vaccination 
  is 
  to 
  cause 
  a 
  very 
  mild 
  

   attack, 
  which 
  produces 
  immunity 
  against 
  fully 
  virulent 
  small- 
  

   pox. 
  This 
  lasts 
  for 
  a 
  period 
  of 
  years 
  (usually 
  given 
  as 
  seven 
  

   years), 
  though 
  the 
  protective 
  effect 
  gradually 
  diminishes. 
  

  

  1 
  The 
  specific 
  organisms 
  which 
  produce 
  smallpox 
  have 
  not 
  been 
  fully 
  

   identified. 
  It 
  is 
  thought 
  by 
  some 
  that 
  smallpox 
  is 
  caused 
  by 
  an 
  animal 
  

   organism 
  somewhat 
  like 
  the 
  malarial 
  parasite, 
  and 
  not 
  by 
  bacteria. 
  

  

  