76 



SCIENCE 



[N. S. Vol. XLIV. No. 1125 



appear. In not a few instances, paralytic 

 diseases among poultry or pigs have been 

 noted to coincide with the appearance of 

 cases of infantile paralysis on a farm or in 

 a community. Experimental studies have, 

 however, excluded the above-mentioned 

 animals from being carriers of the virus of 

 infantile paralysis. The paralytic diseases 

 which they suffer have long been known and 

 are quite different from infantile paralysis. 

 Their occurrence may be coincidental; in 

 no instance investigated has one been found 

 to be responsible for the other. 



Eoutes of Travel 

 Studies carried out in various countries 

 in which infantile paralysis has been epi- 

 demic all indicate that, in extending from 

 place to place or point to point, the route 

 taken is that of ordinary travel. This is 

 equally true whether the route is by water 

 or land, along a simple highway or the line 

 of a railroad. In other words, the evidence: 

 derived from this class of studies confirms 

 the evidence obtained from other sources 

 in connecting the distributing agency inti- 

 mately with human beings and their activ- 

 ities. 



Survival of the Virus in the Infected Body 

 The virus of infantile paralysis is de- 

 stroyed in the interior of the body more 

 quickly and completely than, in some in- 

 stances, in the mucous membrane of the 

 nose and throat. It has been found in 

 monkeys, in which accurate experiments 

 can be carried out, that the virus may dis- 

 appear from the brain and spinal cord 

 within a few days to three weeks after the 

 appearance of the paralysis, while at the 

 same time it is still present upon the muc- 

 ous membranes mentioned. The longest pe- 

 riod after inoculation in which the virus 

 has been detected in the mucous membrane 

 of the nose and throat of monkeys is six 

 months. It is far more difficult to detect 



the human than the monkey carriers of the 

 virus since, as directly obtained from hu- 

 man beings, the virus displays a low degree 

 of infectivity for monkeys; while, once 

 adapted to monkeys, the virus becomes in- 

 credibly active, so that minute quantities 

 are capable of ready detection by inocula- 

 tion tests. Yet in an undoubted instance 

 of the human disease, the virus was detected 

 in the mucous membrane of the throat five 

 months after its acute onset. Hence we 

 possess conclusive evidence of the occur- 

 rence of occasional chronic human carriers 

 of the virus of infantile paralysis. 



Fluctuation in Epidemics 

 Not all epidemics of infantile paralysis 

 are equally severe. Indeed great varia- 

 tions or fluctuations are known to occur 

 not only in the number of cases, but also 

 in the death rate. The extremes are repre- 

 sented by the occasional instances of in- 

 fantile paralysis known in every consider- 

 able community and from which no exten- 

 sion takes place, and the instances in which 

 in a few days or weeks the number of 

 cases rises by leaps and bounds into the 

 hundreds, and the death rate reaches 20 

 per cent, or more of those attacked. "While 

 all the factors which determine this dis- 

 crepancy are not known, certain of them 

 have become apparent. A factor of high 

 importance is the infective power or po- 

 tency, or technically stated the virulence, 

 of the microorganism or virus causing the 

 disease. This virus is subject to fluctua- 

 tions of intensity which can best be illus- 

 trated by an example. The virus as ordin- 

 arily present in human beings even during 

 severe epidemics has low infective power 

 for monkeys. But by passing it from 

 monkey to monkey, it tends to acquire 

 after a variable number of such passages 

 an incredible activity. However, occa- 

 sional samples of the human virus refuse 

 to be thus intensified. But once rendered 



