438 HUMAN BIOLOGY 



influenza, recurring outbreaks may be anticipated at 

 intervals of thirty-three weeks, provided that the thirty- 

 third week does not fall between June and December. 

 Measles tends to recur every two years in large communities 

 or three years in small communities, apparently depending 

 on the accumulation of susceptible children. Protective meas- 

 ures, such as education of the public to pay special attention 

 to colds in children under three years of age and to call in 

 medical men at once, to segregate all contacts of cases, and 

 the offer to immunize young and delicate children by 

 the injection of the blood serum from a convalescent patient, 

 can then be taken to prevent the spread of disease and min- 

 imize the mortality. The study of epidemics of infectious dis- 

 eases artificially induced and experimentally controlled in 

 laboratory animals carried out by Simon Flexner at the Rock- 

 efeller Institute in America and Topley in England may be 

 confidently expected to throw much further light on the fac- 

 tors influencing the occurrence of epidemics, and thus to point 

 the way to their prevention. This again is an example of the 

 value of animal experiments as a means of providing the 

 knowledge necessary to diminish human disease and suffering. 

 Protective Vaccination against Diseases. Edward Jenner's 

 discovery in 1798 that smallpox, a disease which in the past 

 levied a heavy toll in death and disfigurement, could be 

 prevented by introducing under the skin material obtained 

 from cows with the modified disease of cowpox (vaccinia) 

 was a great improvement on the previous method of inducing 

 an attack of smallpox by inoculation of healthy persons with 

 material from patients suffering from actual smallpox in the 

 hope that by producing a mild attack further risk from a 

 virulent form would be obviated. Jenner's discovery laid 

 the foundation of protective immunization against infectious 

 disease; this method arouses the same defensive mechanisms 

 that ordinarily follow a non-fatal attack of the disease 

 but without actually making the inoculated person ill with 

 the ordinary manifestations ot that disease. Such active 

 immunization was employed with complete success by Pasteur 

 in the prevention of hydrophobia in human beings who had 

 already been bitten by a mad dog, and would after a latent or 

 incubation period have developed this constantly fatal 



