One picture holds both uses: the mind has come off its track. Thoughts that normally run in a straight line are jumping about, names come out wrong, and the person cannot steer. What knocked them off the track is what changes — a fever in one use, happiness in the other — but the off-the-track feeling is the same.
可以想成腦子暫時脫離了軌道。本來一條線走下去的思緒開始亂跳，名字叫錯，人也控制不住自己。把人撞離軌道的東西不一樣——一邊是高燒和重病，一邊是太強烈的快樂——但「出了軌」的感覺是同一個。也因為是這個畫面，兩個用法都帶著「暫時的」味道：燒退了會清回來，慶祝過了也會平靜下來。
From the word’s history
The word goes back to Latin dēlīrō, built from dē- 'away from' and līra, the straight furrow a plough cuts in a field. The literal picture is an ox walking out of the furrow, which is remarkably close to what the word still describes.
這個字回到拉丁文 dēlīrō，由 dē-（離開）加上 līra（犁田犁出來的直溝）組成。字面畫面是犁田時走出了那條直溝。這條線索跟今天的意思接得很近：思緒離開了它該走的那條線。
Wiktionary, "delirium"
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