寄生虫

jìshēngchóng - parasite

Despite doing my PhD on a parasite (Trypanosoma brucei), I've never previously wondered how to say parasite in Chinese. It initially seemed a strange word as I know 奇 as to send (a letter), but it also means to live in. 寄生 therefore means to live inside another organism or parasitism; 虫, though normally translated as insect, seems to mean any sort of unpleasent organism.

那懒汉是他家的一个寄生虫。
The lazy man was a parasite on his family.