246 INFECTION AND IMMUNITY 



during the summer months, wherever there are suit- 

 able breeding-places for mosquitoes of the genus 

 Anopheles. 



The more severe and fatal forms of malarial infec- 

 tion are found especially in low-lying regions in the 

 tropics. In the United States malarial fevers are 

 common along the Gulf and South Atlantic coasts, 

 and in the valleys of rivers throughout the Southern 

 and South-western States. There is considerable 

 malaria along the shores of lakes Ontario and Erie, 

 but fevers of this class are rare in the vicinity of lakes 

 Superior and Michigan. The disease prevails in a 

 mild form in some portions of the States of New 

 York and Pennsylvania but is extremely rare in New 

 England and is almost unknown on the Pacific Coast. 

 Many localities in the United States which furnished 

 numerous cases of malarial fever when first settled 

 have since become comparatively healthy as a result 

 of agricultural operations by which marshy lands have 

 been drained and reclaimed. 



The seasonal prevalence differs in different regions, 

 but in the temperate zone is usually greatest during 

 the summer and early autumn. A considerable rain- 

 fall during the spring and summer months leads to 

 the formation of stagnant pools and marshes which 

 serve as breeding-places for mosquitoes, but an ex- 

 cessive rain-fall, by which swamps and ponds are kept 



