CONTAGIOUS EPITHELIOMA 495 
dence to believe that nasal roup and chicken pox are two distinct 
diseases. They further state that immunity to chicken pox does not 
confer immunity to roup. Mack concludes that further investigation 
is necessary to determine the identity or non-identity of these infec- 
tions. 
History. The etiology of chicken pox was found in 1902 by Marx 
and Stricker to be a filterable virus. Since that time a large number 
Fig. 118. A PHOTOGRAPH OF THE HEAD OF A FOWL SHOWING EPITHELIOMA 
ON THE COMB AND IN FRONT OF THE EYE (after Ward). 
of investigations have been made in connection with it some of which 
tend to show, as stated above, that it is identical with avian diphtheria 
and others that it is an independent disease. 
Geographical distribution. This affection seems to be wide-spread 
existing in practically all countries where fowls are kept. It is stated 
(by those who believe chicken pox and roup to be identical) that the 
diphtheritic form is found more frequently in the north and that the 
