The anopheles mosquito is rarely active 1,000 meters or more above sea level, and those villages found below that elevation are mostly located away from rivers and streams, where the mosquitoes breed.
因為傳播瘧疾的瘧蚊，多在海拔一千公尺以下活動，聚落遂往高山發展。至於低於一千公尺的村落，也多遠離河流，這顯然造成飲水不便，有違一般選定居地的原則，應該也是為遠離河流孳長的瘧害。