OBSCURE AND INVISIBLE PARASITES 169 



hind the screens are brought to light, experimented with, and 

 brought under control but there are still some which have defied 

 the most ardent researches of modern science and have never yet 

 been discovered. The fact that many of them are able to pass 

 through filters of certain kinds, as shown by the infectiveness of 

 fluids containing them after having been passed through the 

 filters, demonstrates that at least in some stages of their de- 

 velopment they are actually too small to be visible under the 

 highest power of the microscope. 



However, in the case of some of these unseen parasites we have 

 sufficient knowledge of their habits and life histories to wage a 

 fairly intelligent war against them, at least as regards prevention. 

 The parasite of yellow fever, for instance, was not discovered 

 until 1918. Yet we knew much about its probable nature, we 

 knew its full life history in a general way, and to a large extent 

 we knew how to combat it, far better, in fact, than we know how 

 to combat some of the well-known parasites. There are two 

 other diseases, dengue and phlebotomus fever, which are prob- 

 ably caused by parasites related to that of yellow fever, but they 

 have not yet been discovered. 



The organisms of typhus fever, trench fever, spotted fever 

 and kedani have only recently been discovered. They are all 

 minute bodies showing various morphological types, which occur 

 in both vertebrate and arthropod hosts. They are here grouped 

 together as " Rickettsia-like organisms." Their affinities, whether 

 with bacteria or protozoa, are at present entirely problematical, 

 but for the present they may best be considered, like the spiro- 

 chaetes, to represent a more or less intermediate group. 



Several other diseases, some of them of prime importance, of 

 which the parasites are of obscure nature, are believed by some 

 workers to be caused by Protozoa: such are hydrophobia or 

 rabies, trachoma, smallpox, verruga peruviana (not Oroya fever), 

 foot-and-mouth disease, measles, scarlet fever and a few others. 

 The parasites or parasite-like bodies which are associated 

 with these diseases are in some cases minute, in other cases, 

 e.g., hydrophobia, of relatively large size. In most of these 

 diseases the " germ " or virus is capable of passing through 

 ordinary bacterial filters, as shown by the infectiveness of filtered 

 material. It is also evident from this that the viruses live out- 



