zì rán
不经人力干预（区别于“人工、人造”）：自然免疫
bù jīng rénlì gānyù (qūbié yú 'réngōng, rénzào'): zìrán miǎnyì
without human intervention (as opposed to 'artificial, man-made'): natural immunity
听其自然
tīng qí zìrán
let nature take its course
自然而然
zìrán ér rán
naturally; as a matter of course