VIRULENCE 227 



much attention of recent years because of the extent to 

 which it has lowered the efficiency of the population in 

 those regions where it is abundant. The organism is 

 present in large numbers in the intestinal tract of affected 

 persons, and is expelled with their excreta. Where sani- 

 tary precautions are not observed and the excreta are 

 allowed to become mixed with the soil, these organisms 

 are present in abundance at the surface of the ground. 

 When one walks with bare feet over such an infected 

 spot, some of them are likely to penetrate the skin of the 

 sole, whence they work their way upward among the body 

 tissues, and ultimately to the alimentary tract from which 

 they will be expelled in course of time, to serve as possible 

 sources of further infection. From the above description 

 it is evident that the hookworm can flourish only in re- 

 gions where the simplest sanitary precautions are neg- 

 lected, and where the population is either too poor or too 

 slothful to wear shoes. It should be noted, however, that 

 the chief manifestation of hookworm infection is marked 

 bodily lassitude, so that the lack of sanitary precautions 

 is in part due to the lowering of vitality consequent on 

 the disease. Fortunately for the welfare of those dis- 

 tricts, including parts of the United States and large areas 

 in Central and South America, where hookworm infesta- 

 tion occurs, quite simple treatment with the drug thymol, 

 combined with simple sanitary regulation, suffices to 

 stamp out the parasite promptly and effectively. 



Virulence. — A fact that must be taken into account 

 in any attempt to understand the course and spread of 

 infectious disease is that its producing organisms may dif- 

 fer greatly from time to time in what is called virulence. 

 Under certain circumstances the organisms, although pres- 

 ent, may produce only few cases of disease and these 

 mild. Under certain other circumstances nearly every- 

 one exposed may be infected and the individual cases 

 may be very serious. These differences are undoubtedly 

 due to alterations in the metabolism of the disease-pro-. 



