Webster's Revised Unabridged Dictionary · 1913

Ail

Ail (āl) , transitive verb

[Old English eilen, ailen, Anglo-Saxon eglan to trouble, pain; akin to Gothic us-agljan to distress, agls troublesome, irksome, aglo, aglitha, pain, and prob. to English awe. r3.]

To affect with pain or uneasiness, either physical or mental; to trouble; to be the matter with; -- used to express some uneasiness or affection, whose cause is unknown; as, what ails the man? I know not what ails him.
What aileth thee, Hagar? — Gen. xxi. 17

It is never used to express a specific disease. We do not say, a fever ails him; but, something ails him.

Ail , intransitive verb

To be affected with pain or uneasiness of any sort; to be ill or indisposed or in trouble.
When he ails ever so little... he is so peevish. — Richardson

Ail , noun

Indisposition or morbid affection.