2 INTRODUCTION 



One by one the diseases which formerly held the world in terror, 

 or made parts of it practically uninhabitable, are falling before 

 the onslaught of modern science. The vast majority of human 

 and animal diseases are now known to be caused by organisms 

 which live as parasites within the body. In all but a few cases 

 these organisms are now definitely known, their habits under- 

 stood, their means of transmission and multiplication worked 

 out. What such knowledge means to the human race can hardly 

 be overestimated. In the 14th century Europe was swept by an 

 epidemic of plague which destroyed probably one-fourth of her 

 entire population — something like 25,000,000 people. That 

 a similar epidemic would have swept over the United States 

 in the present century had it not been for modern scientific 

 knowledge of the cause and means of transmission of plague, 

 which made it possible to nip the epidemic in the bud in San 

 Francisco and New Orleans, is reasonable to believe. In the 

 latter part of the 19th centmy the French attempt to build a 

 canal at Panama failed dismally after a stupendous loss of life 

 from yellow fever and malaria. Shipload after shipload of 

 laborers, engineers, nurses and doctors were sent to the great 

 " white-man's graveyard," the majority to succumb in a few 

 weeks or months to these diseases, at that time uncontrollable. 

 In the early part of the 20th century, by exterminating malaria 

 and j^ellow fever on the Canal Zone, through the application of 

 the knowledge which had been gained in the intervening 3'ears, 

 the Americans made possible the building of the Panama Canal. 

 In an incredibly short time this zone was transformed from a 

 veritable pest hole to one of the healthiest places in the world, 

 and incidentally the " conquest of the tropics," previously looked 

 upon as a more or less hopeless task, was shown to be not only 

 possible but profita])le. To quote another example, through- 

 out the history of the world typhus fever has hovered like a 

 death dragon over nearly every army camp ever assembled, 

 and has followed in the wake of war to add the last touch of 

 horror and desperation to the inhabitants of the countries involved. 

 In the present unprecedented war only those countries which have 

 not kept abreast of the times in the application of scientific knowl- 

 edge have suffered seriously from this terrible scourge. Were it 

 not for the application of modern knowledge the horrors of the 

 present war would have been even more awful than they are now. 



