AN INSECT-BORNE DISEASE 231 



children in one family were taken with it and died. There were children in 

 almost every house and the parents felt the gravest alarm. Many were 

 physicians accustomed to control disease and not to fear it. Here was an 

 unknown force however which no one could control. Was this disease 

 carried by people or air or domestic animals or insects or food supplies? 

 No one knew. Finally a fourth case occurred, further along the road; and 

 the next morning every family in the little colony had gone. There was no 

 rational mode of fighting this "pestilence that walketh in darkness," no 

 recourse but flight before it. This was the attitude of mankind toward 

 every epidemic disease before Jenner and Pasteur. 



The first steps were taken in clearing up the mystery of infant paralysis 

 when in 1908 Landsteiner and Popper in Vienna succeeded in transmitting 

 the disease to monkeys. Flexner and Lewis at the Rockefeller Institute in 

 the next year extended this work and proved by successive inoculation ex- 

 periments that there was a living germ present in the body and in the nose 

 and throat of affected persons although this germ belongs to the class of 

 the "filterable viruses," organisms so minute that they will pass through 

 the pores of a Berkefeld filter and cannot usually be distinguished under the 

 most powerful microscopes. Very recently during the past winter, Flexner 

 and Noguchi have added new laurels to the Rockefeller Institute and to New 

 York by cultivating this almost invisible germ outside the body. Animal 

 experimentation however, which has been the chief agent in bringing com- 

 municable disease under control, made it possible much earlier to detect 

 the presence of the germ by its effects. Flexner and Clark at the Rocke- 

 feller Institute, Kling, Wernstedt and Peterrsen at Stockholm, Osgood in 

 Boston, and others showed that the germ may persist in the nose and throat 

 after recovery and that it may be found in the nose and throat of healthy 

 persons who are therefore liable to act as carriers of infection, although 

 themselves not sick. It seemed probable that at least one method of 

 spread of infant paralysis was by more or less direct contact between sus- 

 ceptible persons (particularly children) and either previous cases or healthy 

 carriers. 



There were two things however which seemed puzzling and which could 

 not easily be harmonized with this theory of spread by human contact. In 

 the first place infant paralysis is preeminently a summer disease. Cases 

 do occur at all seasons but the great majority develop during the warm 

 weather. Now this is a characteristic of insect-borne diseases like malaria 

 and yellow fever since the breeding of the insect hosts is directly dependent 

 upon temperature. On the other hand, diseases which spread by contact 

 usually reach their maximum in winter when people are crowded together 

 and there is most opportunit^v for the interchange of germs. There are 

 exceptions to this rule, like typhoid fever, which is normally a summer 

 disease though in the northern United States it is not generally carried by 



