INTRODUCTION TO THE STUDY OF INFECTIOUS DISEASES 381 



and African relapsing fever, transmitted by a species of tick, 

 European and Balkan relapsing fever, and kala-azar transmitted by 

 bed bugs, and typhus fever, trench fever and some varieties of 

 relapsing fever transmitted by lice, and plague largely transmitted 

 by fleas. 



In the sanitation of these diseases it is quite obvious that, in 

 addition to our knowledge of the pathogenic microorganisms which 

 cause the disease, we must study carefully the habits of insects, 

 their seasonal occurrence, their nocturnal or diurnal habits, their 

 methods and places of breeding, the distances which they can travel, 

 the manner in which they acquire the parasites and the manner in 

 which they can be suppressed. 



Based upon an understanding of the conditions outlined above, 

 moreover, is the science of epidemiology which, in its turn, can in- 

 directly contribute a great deal, both to the solution of the problems 

 of bacteriology and to those of transmission. For, by epidemiological 

 surveys, by a careful study of the manner of spread of disease 

 from group to group, and from place to place, the explosiveness 

 with which it appears and relationship of cases to personal contact, 

 water or milk supplies, etc., many deductions can be made which 

 have important bearing in guiding bacteriological investigation and 

 preventive measures. As to preventive measures, nothing is so im- 

 portant in the suppression of an epidemic disease as the rapid 

 circumscription of the initial focus, a thing which can be accom- 

 plished only by prompt epidemiological survey, backed up by 

 accurate bacteriological diagnosis. Such results can be achieved 

 only when communities have efficient organizations for the prompt 

 reporting of communicable diseases, for the systematic charting of 

 the cases and for intelligent and experienced study of such charts. 

 Indeed, studies of this nature alone may often make possible a 

 causal classification of the epidemic before its bacteriological nature 

 is accurately known. For epidemics will vary in the respects men- 

 tioned above, very largely according to whether they are water 

 borne, distributed by milk or other food, or whether they are spread 

 by contact or by insects. 



It is probable that small water epidemics from individual rural 

 house supplies may still be quite frequent and simulate contact 

 epidemics. The larger water epidemics of typhoid, cholera, etc., will 

 become, as we have stated, more and more infrequent as water 

 supplies are more carefully supervised, but when they do occur, 



