6 INFECTION AND IMMUNITY 



does not multiply in the body of the individual bitten 

 and the symptoms produced bear a direct relation to 

 the amount injected. Moreover, the symptoms fol- 

 low very closely after the bite. On the other hand, an 

 infectious disease resulting from inoculation, or con- 

 tracted in any other way, is not developed at once ; 

 but after the introduction of the infectious material a 

 certain interval elapses, technically known as the 

 "period of incubation," before the symptoms char- 

 acteristic of the disease are manifested. In the case of 

 hydrophobia, resulting from the bite of a rabid animal, 

 this so-called period of incubation may be greatly 

 prolonged. It is seldom less than two weeks and may 

 be six months or more. But, as a rule, the period of 

 incubation is quite definite for each infectious dis- 

 ease, although differing greatly in different diseases. 

 Thus it is usually less than three days in scarlet fever 

 and diphtheria ; from two to five days in yellow fever 

 and influenza ; from seven to ten days in whooping- 

 cough ; eleven or twelve days in smallpox ; fourteen 

 days in measles ; and from seventeen to twenty-one 

 days in mumps. In wound infections, resulting from 

 the introduction of certain well-known disease germs 

 into wounds produced by the surgeon's knife or other- 

 wise, the period of incubation is comparatively short, 

 and erysipelas or " blood-poisoning " may be developed 

 within a few hours after the inoculation occurs. 



