The United States was not fighting a specific group, making enemy identification difficult since soldiers were constantly attacked by guerrilla insurgents (the Viet Cong).
Countries like the Philippines and the Dominican Republic and Viet Nam, which don't have U.S. -type resources, had much lower infection and death rates during the time of the study.
That meant that soldiers were removed as soon as they understood the military situation, so the American soldiers on the ground were usually unprepared and inexperienced in fighting the Viet Cong.