INTRODUCTION TO THE STUDY OF INFECTIOUS DISEASES 383 



of recently occupied territory, the prevalence of flies, and the crowd- 

 ing of large masses of men in small areas made frequent contact 

 unavoidable. Such conditions, however, are not likely to happen 

 to any extent at ordinary times, and contact epidemics are usually 

 limited in area and distribution, and careful epidemiological study 

 of three or four cases may lead to the original focus which can then 

 be determined by prompt laboratory investigation. 



