July 21, 1916] 



SCIENCE 



77 



highly potent, the virus may be passed 

 from monkey to monkey through a long 

 but not indefinite series. Finally, in some 

 samples of the virus at least a reverse 

 change takes place — the virus begins to 

 lose its virulence until it returns to the 

 original or even to a diminished degree of 

 infective power. In this respect the be- 

 havior of the virus corresponds to the on- 

 set, rise and then the fall in number and 

 severity of cases as observed in the course 

 of epidemics of infantile paralysis and 

 other epidemic diseases. Hence either a 

 new active specimen of the virus may be 

 introduced from without which, after a 

 certain number of passages from person to 

 person, acquires a high potency ; or a speci- 

 men of virus already present and left over 

 from a previous epidemic after a resting 

 period and similar passages, again becomes 

 active and reaches an infective power which 

 equals or even exceeds that originally pos- 

 sessed. Another but more indefinite factor 

 relates to the degree of susceptibility 

 among children and others affected which 

 at one period may be greater or less than 

 at another. 



Varying Individual Susceptibilities 

 Not all children and relatively few 

 adults are susceptible to infantile paraly- 

 sis. Young children are more susceptible 

 generally speaking than older ones ; but no 

 age can be said to be absolutely insuscep- 

 tible. "When several children exist in a 

 family or in a group, one or more may be 

 affected, while the others escape or seem 

 to escape. The closer the family or other 

 groups are studied by physicians, the more 

 numerous it now appears are the number 

 of cases among them. This means that the 

 term infantile paralysis is a misnomer, 

 since the disease arises without causing 

 any paralysis whatever, or such slight and 

 fleeting paralysis as to be difficult of detec- 

 tion. The light or abortive cases, as they 



are called, indicate a greater general sus- 

 ceptibility than has always been recog- 

 nized; and their discovery promises to 

 have far-reaching consequences in respect 

 to the means employed to limit the spread 

 or eradicate foci of the disease. 



Period of Incubation 

 Like all other infectious diseases, in- 

 fantile paralysis does not arise at once 

 after exposure, but only after an interven- 

 ing lapse of time called the period of incu- 

 bation. This period is subject to wide lim- 

 its of fluctuation : in certain instances it 

 has been as short as two days, in others it 

 has been two weeks or possibly even longer. 

 But the usual period does not exceed about 

 eight days. 



Period of Infectivity 

 Probably the period at which the danger 

 of communication is greatest is during the 

 very early and acute stage of the disease. 

 This statement must be made tentatively 

 since it depends on inference, based on 

 general knowledge of infection, rather 

 than on demonstration. Judging from ex- 

 periments on animals, the virus tends not 

 to persist in the body longer than four or 

 five weeks except in those exceptional in- 

 stances in which chronic carriage is de- 

 veloped. Hence cases of infantile paraly- 

 sis which have been kept under super- 

 vision for a period of six weeks from the 

 onset of the symptoms may be regarded 

 as practically free of danger. 



Protection by Previous Attack 

 Infantile paralysis is one of the infec- 

 tious diseases in which insusceptibility is 

 conferred by one attack. The evidence de- 

 rived from experiments on monkeys is con- 

 clusive in showing that an infection which 

 ends in recovery gives protection from a 

 subsequent inoculation. Observations upon 

 human beings have brought out the same 



