One way to picture ail: something is quietly wearing a person down. They are not fine, they have not been fine for a while, and nobody can point at the exact cause. That is different from a broken arm or a bad cold, which have names. The word keeps that shape wherever it goes: whatever ails you, and whatever ails a school system or a company, is inside, has been there some time, and is still waiting to be named.
From Middle English eilen, from Old English eġlan 'to trouble, afflict', from Proto-Germanic *aglijaną 'to trouble, vex'. From the start the word named the troubling, not a particular disease, which is why the modern uses still feel undiagnosed.
來自中古英文 eilen，再往上是古英文 eġlan「使人苦惱、折磨」，源頭是原始日耳曼語 *aglijaną「使煩擾」。這個字從一開始講的就是「有東西在折磨」，而不是某一種特定的病——今天的用法之所以還帶著「說不清楚是什麼」的感覺，原因就在這裡。
Wiktionary entry for ail (etymology 1) supplied in this packet (source_ref kaikki:3f6f69a190036ae4:163412319:12586)
Two branches
人不舒服、久病
這一支最接近字的原始意思：被折磨的是一個人的身體。
verb · C1
To be ill or weak, usually for a long time, or to make someone feel ill or unhappy. This is an old-fashioned word: people today normally say be ill or be sick, but you still meet ail in older books and in questions like what ails you?
生病、不舒服
制度與組織的病根
同一個畫面換一個對象：被磨著的不再是身體，而是一間公司、一套制度、一個城市。
verb · C1
Used in the phrase what ails someone or something to talk about the problems that are making a person, a company, a system or a country weak or unhealthy. It treats the trouble like an illness that has not been given a name yet.
問題出在哪裡
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
ailverb詞源 1/ˈeɪ̯l/
To cause to suffer; to trouble, afflict. (Now chiefly in interrogative or indefinite constructions.)
To be ill; to suffer; to be troubled.
ailnoun詞源 1/ˈeɪ̯l/
obsoleteAn ailment; trouble; illness.
ailadj詞源 2/ˈeɪ̯l/
obsoletePainful; troublesome.
ailnoun詞源 3/ˈeɪ̯l/
The awn of barley or other types of corn.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。