CHAPTER XV 

 DISEASES OF UNKNOWN ETIOLOGY 



WHILE this book concerns itself with the relation 

 of microorganisms to disease, it is fitting that men- 

 tion be made of some communicable affections, in 

 which the causative agent is not yet known. The 

 clinical observations upon these infections indicate 

 that they are due to some form of living body which 

 present methods of investigation do not permit us to 

 demonstrate. It is inconceivable that so specific a 

 condition as smallpox should come from anything but 

 a self reproducing agent. 



Smallpox or Variola. This is an acute infectious dis- 

 ease characterized by severe constitutional symptoms 

 and a rash which becomes pustular, leaving behind it 

 after recovery peculiar depressed scars. It is believed 

 today that the various affections of man, cow, horse, 

 and sheep are practically identical. Certain it is that 

 infection with cowpox will give resistance to human 

 smallpox. Vaccination was formerly practised by 

 transferring the pox from person to person, but now 

 fresh material is used from a cow which has been 

 artificially infected with cowpox. Jenner in 1798 was 

 the one who first developed the principle of using 

 cowpox in the protection against human variola. The 

 exact cause of smallpox is not known. It is supposed 



