PAPERS ON MEDICINE AND PUBLIC HEALTH 259 



cation. Epidemic encephalitis is a new disease little 

 known till the last two years. Its long course however 

 makes it a dreaded affliction and its eradication imperative. 

 Trench fever is of little importance in this country in times 

 of peace at least. The common cold is the most costly of 

 all of our diseases. While various factors are concerned 

 in the common cold, a filterable virus has recently been iso- 

 lated as one excitant. It is gratifying to know that in all 

 four of these diseases, priority of investigation as to the 

 real cause belongs to American scientists. No longer are 

 our laboratories dependent upon Europe for stimulation. 



The nature of the filterable viruses as a group is un- 

 certain. Park divides them into three classes; first, dis- 

 eases produced by filterable agents of unknown morpho- 

 logy, an example of which is foot and mouth disease. 

 These organisms are probably too small to be visible with 

 our most powerful microscopes. Second, diseases pro- 

 duced by filterable agents shown to be visible. Polyomyeli- 

 tis comes in this class, the virus of which can barely be 

 seen with a microscope. Third, diseases produced by vi- 

 ruses of questionable filterability. In this class are several 

 diseases, one of which is smallpox. Another classification 

 which might be used is a division into plant and animal 

 kingdoms. In the plant kingdom are those viruses closely 

 related to the bacteria, as polyomyelitis and encephalitis. 

 In the animal kingdom, one disease, the agent of which in 

 certain stages is filterable, has lately been shown to be re- 

 lated to protozoa, namely yellow fever. Noguchi has very 

 lately shown this to be Leptospira icteroides. Probably 

 other diseases, especially those in which mosquitos are in- 

 volved in transmission, belong in the same group. Both 

 Williams and Hawkins have claimed that the virus of rabie,^ 

 belongs among the rhizopods. 



Lately an Italian investigator has shown that any sub- 

 stance which will cause the clumping of the viruses makes 

 these organisms easy to study with the microscope. Great 

 possibilities lie in this field. 



There are still many problems to be solved. One that is 

 to be emphasized especially is that more attention be paid 

 to this group of organisms. There should be devised more 



