CHAPTER XV. 

 DISEASES OF UNKNOWN ETIOLOGY. 



WHILE this book concerns itself with the relation of micro- 

 organisms to disease, it is fitting that mention 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 disease 

 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 arti- 

 ficially infected with smallpox. By passing this virus through 

 the calf it is so altered that it cannot produce smallpox in 

 man, yet it can, when inoculated into the skin, call forth an 



(199) 



