Something harmful has already got inside — on a knife, a virus in a file — and this word is the act of going in after it and killing it. Not tidying, not throwing the whole thing away: hunting down the small living thing that could spread, and leaving the object itself still usable.
可以想成有壞東西已經跑進去了——刀子上的細菌、檔案裡的病毒——而這個字就是進去把它殺掉的動作。重點不是整理，也不是整個丟掉，而是把那個會擴散的小東西找出來清乾淨，東西本身還能繼續用。所以英文才會說 dis-（去除）加 infect（感染）。
From the word’s history
The packet's Wiktionary etymology gives dis- + infect, alongside Middle French desinfecter. Infect is the harm getting in; dis- undoes it. The modern meaning is still exactly that shape.
來源記錄的字源就是 dis- 加 infect，並對照中古法文 desinfecter。infect 是壞東西進去，dis- 把它倒過來。今天的意思和這個組合完全一樣，所以這個字很好拆。
Wiktionary etymology supplied in packet (kaikki:3f6f69a190036ae4:1220630381)
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