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, has never been seen 

 with certainty. Yet we know almost beyond question that it is 

 a protozoan, we know its full life history in a general way, and to 

 a large extent we know 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 quite certainly caused by parasites related to that of 

 yellow fever, but which have not yet been discovered. Until 

 recently typhus fever was included in the list of possible proto- 

 zoan parasites but Plotz in 1914 discovered a bacillus which is 

 now quite generally believed to be at least partially the cause 

 of that disease. Rocha-Lima and others have found certain 

 minute bodies in typhus-infected lice which they suspected might 

 be of protozoan nature, and Rocha-Lima has named them 

 Rickettsia prowazeki.* American investigators are inclined to 

 look upon these bodies as forms of the bacillus discovered by 

 Plotz. 



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- 

 * See footnote on p. 73. 



