NATURAL IMMUNITY 55 



ported cultivation of the organism of the disease by Plotz is authen- 

 ticated. 



Yellow fever up to the present has been observed in man only. 



Poliomyelitis is spontaneous in man only. Can be transmitted to 

 monkeys and in a doubtful form to rabbits. No other animals 

 are known to be susceptible. 



The above represents an incomplete tabulation of the variations 

 in susceptibility in the animal kingdom for infections which occur 

 spontaneously in man. They will illustrate sufficiently, however, 

 the facts of variable species susceptibility as we have stated them. 

 We might, with equal profit, tabulate the infections occurring spon- 

 taneously in any single species of animal and show how variable 

 would be their pathogenic powers for other animals and for man. 

 Thus man is immune to the organism which causes cattle plague, 

 and to that of chicken cholera, and probably to many other diseases 

 peculiar to animals, though, of course, in the case of infections of 

 the human being we are entirely dependent for such information 

 upon observed immunity to spontaneous infection, and upon a few 

 instances of accidental inoculation. 



In regard, also, to differences of susceptibility between various 

 races, within the same species, many interesting facts have been ob- 

 served. Thus gray mice are, as a rule, more resistant to strepto- 

 coccus and pneumococcus infection than are wnite mice. Algerian 

 sheep are said to be more resistant to anthrax than are European 

 sheep. Of black rats inoculated by Miiller 4 with anthrax over 79 

 per cent, survived, while of white rats similarly inoculated only 14 

 per cent, survived. 



In man, too, racial differences are marked. The extraordinary 

 susceptibility of the negro to tuberculosis is familiar to all American 

 physicians, and it is well known that Eskimos transported to tem- 

 perate climates and civilized conditions are particularly prone to 

 contract this disease. Small-pox is considered a relatively mild 

 disease in Mexico. Dr. James Carroll 5 stated that whites are more 

 susceptible to yellow fever than are negroes, and that among the 

 latter those living nearest the equator are less susceptible than are 

 the more northern races. There seems to be no doubt about the 

 actual occurrence of such racial differences, although, as Hahn 6 

 very justly points out, many instances formerly regarded as racial 

 differences of susceptibility may have been simulated by racial, 

 or often religious, differences of custom that influence sanitary con- 

 ditions, and consequently the incidence of epidemic disease. 



Apart from the explanations furnished in a few instances by 



4 Miiller. Fortschr. der Med., 1893. Cited from Sobernheim, in "Kolle 

 u. Wassermann Handbuch," 2d Ed., Vol. 3. 



5 Carroll in "Mense, Tropenkrankheiten," Vol. 2, p. 124. 



6 Hahn in "Kolle und Wassermann's Handbuch," Vol. 1. 



