TL;DR: By spraying DDT everywhere, and coercing troops to take Atabrine (that often had nasty side effects), they reduced malaria incidence by about 70%. The Japanese, by contrast had little protection - nearly all Japanese soldiers had malaria at some time.
https://armyhistory.org/the-other-foe-the-u-s-armys-fight-ag...
TL;DR: By spraying DDT everywhere, and coercing troops to take Atabrine (that often had nasty side effects), they reduced malaria incidence by about 70%. The Japanese, by contrast had little protection - nearly all Japanese soldiers had malaria at some time.