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. Nevertheless the viruses of 

 these diseases must be, at least in some part of their 

 existence, very tiny, because they are able to pass 

 through the pores of a porcelain filter that would hold 

 back bacteria. For this reason the following diseases 

 are said to be due to "filterable viruses." We may 

 later learn to know the agents as physical entities, but 

 those which can be cultivated now are only imperfectly 

 understood. 



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 



